* KarmaHoudini:
** [[LampshadeHanging Lampshaded]] in the episode where Leonard meddles in Penny's date with Stuart.
** Penny's ex boyfriend Kurt. In the Pilot, Kurt split up with Penny and got to keep their television set. When Leonard and Sheldon attempt to take the TV, Kurt strips them off their pants. He gets away with it. In the later episode "The Financial Permeability", Kurt is arrested for urinating on a police car. When arrested, it turns out he had a slew of unpaid tickets, as well as an outstanding bench warrant. Penny pays all his fines and Kurt does not intend to repay her. Months later, Leonard confronts him about this, and Kurt writes an "I owe her" note...on Leonard's forehead in indelible ink. Kurt does pay Penny back, but never reveals Leonard's involvement. So Penny is grateful to Kurt and they have a dinner date. Kurt faces no consequences for his behavior.
** Creator/WilWheaton is often this. His debut appearance ends with him winning some money [[BatmanGambit for pulling an underhanded sympathy ploy on Sheldon]].
** Sheldon is this in "The Weekend Vortex." He made plans with Amy to visit her family, and even though the Relationship Agreement requires her to do a ridiculous amount of things before he would agree such as 72 hours notice, checking the tire pressure on her car, and calling the CDC to see what kind of inoculations are required for Orange County (there aren't any). he decides not to go at the last minute instead opting to play video games all weekend. When Amy and Penny go to confront Sheldon, the episode turns into how lonely Raj is causing all the woman to leave and Sheldon to get off scot free.
** Sheldon in general. He throws giant whiny tantrums to get his own way. Sometimes he gets LaserGuidedKarma for this, but sometimes it ends with the others giving in to shut him up.
* KidsPreferBoxes: Leonard jokes about this in a Season 6 episode.
-->'''Penny:''' ''(referring to a package from Sheldon's mother)'' So what's in it?\\
'''Leonard:''' Doesn't matter. Half the time he just ends up playing with the box.
* KilledOffScreen: The character Debbie Wolowitz dies off screen and out of sight of her son Howard. She was visiting relatives in Florida. Howard receives a phone call alerting him of her death.
* KillerGorilla: The eponymous villain of the (fictional) BMovie ''Serial Ape-ist'' is a sex-crazed killer gorilla. In his one known scene, the gorilla kills a nude blonde woman (played by Penny).
* KillerRobot: "MONT-E" whom the guys built for a robot fighting competition.
** Kripke's robot makes MONT-E look like a children's toy
* KissingCousins: Howard and his cousin Jeannie, apparently.
* KitschCollection: The boys' compulsive accumulation of tacky sci-fi, fantasy and comic-book tie-ins could be viewed in this light. When Penny criticises Leonard for this and he is moved to sell his collection off to please her, Sheldon calls her about her accumulation of Care Bears, My Little Ponies, and other feminine-marketed kitsch. But the lonely Doctor Lorvis (who tries to romance Penny) tops everyone with ''his'' compensatory collection of high-end desirable kitsch - the Fortress of Solitude in the basement, that helps him escape being single and still living at home with his mother.
* KryptoniteFactor: To resolve a running and escalating argument between Penny and Sheldon -- Sheldon has just distributed her intimate underwear across telephone and power lines in the area -- Leonard Hofstadter gives her the secret of Sheldon's personal weakness and the way she can utterly defeat him. He knocks on her door and gives her a deadly secret. It's the address and phone number of Sheldon's formidable mother.
--> This is Sheldon's Kryptonite. You didn't get it from me.
%% KryptoniteProofSuit
* LaserGuidedKarma: Raj invokes this when Leonard intentionally tries to screw up Stuart's date with Penny;
--> '''Raj''': Leonard pretends to be a friend to Stuart, but instead is a two-faced bitch. Thus, he is reincarnated as a banana slug!
* LateArrivalSpoiler: In trying to comfort Sheldon over a deception the others pulled on him while they spent three months at the North Pole, Penny referred to the '09 ''Film/StarTrek'' film and explained a spoiler-heavy plot point as a comparison. Sheldon had not seen the movie yet. "I missed Comic-Con ''and'' the new ''Franchise/StarTrek'' movie!"
** In "The Spoiler Alert Segmentation": Sheldon spoils plot points from ''Literature/HarryPotterAndTheHalfBloodPrince'' and ''[[Literature/HarryPotterAndTheDeathlyHallows Deathly Hallows]]'' to Leonard who is currently reading ''Half-Blood Prince''; Penny herself accidentally lets out a spoiler from ''Half-Blood Prince'', more than SEVEN and FIVE years respectively after the books were released and a few years after the movies were released.
* LaughTrack: The show does, in fact, employ a live StudioAudience, but as what happens with popular shows is that the audience ends up being filled by big fans excited to see their favorite characters live. This leads to a lot of laughter at the smallest jokes, sometimes just because they know a punchline is around a corner [[note]]such as when Sheldon mentions his replacement laptop has Windows 7, which is more user-friendly than Vista, the audience laughed well before he said "I don't like it."[[/note]]. And like other studio audience shows, it uses what is known as "sweetening" the laughter. Scenes that did not have the right reaction (where the audience laughed too loudly (drowning out the actors), too hard (ruining the comedic rhythm), too soon (preventing people from hearing the joke) or otherwise not reacted how or when they were supposed to [[note]]such as one scene where Penny kisses Leonard, the scene as-aired had no reaction, but the audience kept saying "oooh" on every take[[/note]]) will blend the live audience noise with recorded laughter. In a number of HilariousOuttakes you can see the actors sometimes reacting to the audience response.
* LetsGetDangerous: Sheldon, gets a moment like this in the penultimate episode of Season 3. It's revealed that the reason Leonard puts up with Sheldon is due to the fact that before the series began, [[spoiler: Leonard, Raj, and Howard were messing with some rocket fuel despite Sheldon's objections, which then threatened to explode. As Leonard got into the elevator to take the smoking fuel out of the building, Sheldon calmly entered the elevator, set the canister down on the floor and pushed Leonard out before closing the doors and leaving himself. When Leonard complained that he had plenty of time, the elevator doors exploded outward. Sheldon simply said "You're welcome" and walked back into the apartment.]] This is also why the elevator has not worked since the start of the show.
* LikeAnOldMarriedCouple: Raj and Howard. Lampshaded by Penny and Sheldon in Season 5.
** Heck, lampshaded by Leonard's mom somewhere in Season 2 or 3, when she called it an "ersatz homosexual relationship" created to fill the void left by their repeated failures at maintaining heterosexual ones.
** Leonard and Sheldon are also [[DependingOnTheWriter occasionally portrayed this way.]]
* LegoGenetics: The KillerGorilla in the (fictional) BMovie ''Serial Ape-ist'' was genetically engineered.
* LimitedSocialCircle: The four guys and the three girls. That's pretty much it. Even at Howard and Bernadette's wedding there were only two family members present, and evidently no other friends in the city who were worth inviting to the rooftop ceremony.
** To be fair, the wedding was last minute. Compare the attendance of the wedding versus the attendance at Howard's bachelor party. There were at least 10 people in attendance for the stag party.
* LimitedWardrobe: In a fun mix between this trope and UnlimitedWardrobe, everyone has a series of outfits that they rotate through but are largely on the same theme.
** Sheldon generally wears two t-shirts, the outer shirt usually a superhero symbol like the [[Franchise/TheFlash Flash]] and the under shirt has long sleeves rolled up to his elbows.
** Leonard has a jacket over a hoodie and brown pants he is always wearing. He often wears geeky t-shirt too, but are a little less flamboyant as Sheldon's (such as the chemical composition of caffeine).
** Raj always wears a sweater-vest with a windbreaker jacket.
** Howard wears uncomfortably tight turtlenecks under a button up shirt, odd colored jeans and wacky belt buckles. Oddly enough, among the guys he seems to be the most fashionably conscious and particular about what he wears, but it is still garish. He often wears a alien lapel pin on the turtleneck, which apparently has some meaning but [[WhatsASecretFour won't ever be revealed]].
** Penny has a wider variety, but her leisure clothing also tends to repeat. In the early seasons she was known for [[MsFanservice wearing crop tops, tank tops and short shorts]], by the third season she started wearing slacks and blouses more often.
** Amy wears shapeless brown wool sweaters and long skirts, it's been lampshaded that her fashion sense seems to be 100 years old.
** Bernadette wears button up sweaters and floral print dresses. The color pink tends to always be on her.
* LipstickLesbian: Lakshmi.
* LiterallyLovingThyNeighbor: Penny and Leonard
* LivingInAFurnitureStore: Downplayed with Leonard and Sheldon's apartment, it's immaculately clean but it fits their personality, especially [[NeatFreak Sheldon]]. But the walls and desks are lined with books, computer software and nerdy memorabilia, making it a lot more cluttered than most examples. Penny's apartment, on the other hand, is a lot messier and [[ThePigPen established as such early on in the series]].
* LoanShark: Averted by Sheldon, to the cast's and viewers' surprise. Apparently, loaned money is the only thing which Sheldon is not a neurotic, schedule-obsessed jerkass about, if only because the only money he lends is money he doesn't need for anything in particular.
* LocalHangout: The Cheesecake Factory, largely by virtue that Penny can be involved with the scene while she is working. The Comic Book Shop also comes up in later seasons.
* LockedInAFreezer: One of Sheldon Cooper's more practical and sensible ideas is for the rest of The crew to acclimatise for a trip to the North Pole by spending time inside the lock-up deep freeze room at the Cheesecake Factory. Irked by a passing Sheldon insult, Penny invites him to reflect on the fact the freezer may only be opened from the outside and she might just go a bit deaf...
* LogicBomb: At least for [[DumbIsGood Zack]]...
-->'''Penny:''' I'm not talking to you!
-->'''Zack:''' Then who are you talking to!? ...Babe?
* LonelyBachelorPad: In a flashback to when Leonard became Sheldon's roommate, it's shown that the living room is furnished with just a couple of lawn chairs and a TV.
* LonersWillStayAlone: Subverted by Sheldon Cooper: he has no interest whatsoever in making friends and ''no redeeming personal qualities whatsoever'', yet [[MinimalistCast for some reason]] people keep talking to him.
* LongDistanceRelationship: Leonard and Priya have one in Season 5. It ends in "The Good Guy Fluctuation" (Season 5, episode 7), when he learns that Priya cheated on him.
* LongGame: Sheldon once knew Leonard hadn't returned a DVD to the video store, so he paid for the DVD and let it sit. It took seven years for Leonard to find it and try to make it right.
* LongRunner: Ten seasons so far.
* LookingForLoveInAllTheWrongPlaces: Leonard, at the end of Season 3 and the better part of Season 4.
** Penny in the first two seasons with her wide array of dumb, {{Jerkass}} [[GirlOfTheWeek guys of the week]].
* LoopholeAbuse: Attempted by Sheldon in a game of "Magic Warlords of Ka'a". He plays a card called "Infinite Sheldon" that he claims beats all other cards and does not violate the no homemade cards rule because he "made it at work."
* LoserGetsTheGirl: Leonard at Penny's Halloween party (though he ends up being AboveTheInfluence).
** From the same episode, Raj hooks up with a (admittedly) drunk girl. ''Specifically because he's too afraid to say anything''.
--->'''Drunk Girl''': You're ''such'' a good listener!
* LovableNerd: Leonard and Raj.
%%* LudicrousPrecision

* MadeOutToBeAJerkass: When Penny finally tells Howard that he sucks, he is so upset that he refuses to leave his room or help his friends with their robot for their upcoming battle with Kripke, so Leonard has Penny give an OrderedApology. [[NotListeningToMeAreYou Then he ignores what she says and tries to kiss her]].
* MadonnaWhoreComplex: Amy refers to an outfit she's wearing to a wedding with Leonard as "the perfect mix of Madonna and Whore".
* MadScientist: Sheldon was once described as being "One lab accident away from being a super villain". Among other things, as a child he tried to order uranium to build a reactor, then attempted to build a sonic death ray when a government agent explained that he couldn't buy uranium. Lampshaded and referenced by Sheldon's childhood hero, Arthur Jefferies/Professor Proton (Creator/BobNewhart) when he meets Sheldon.
--> '''Professor Proton:''' Is uh, is he dangerous?
--> '''Leonard:''' Actually he's a genius.
--> '''Sheldon:''' I am.
--> '''Professor Proton:''' That uh, that doesn't answer my question.
** Creator/WilWheaton plays one in the (fictional) BMovie ''Serial Ape-ist 2: Monkey See, Monkey Kill''. His [[LoveInterests Love Interest]] is deceased and the scientist decides to clone her. Only her DNA gets mixed with that of the KillerGorilla who killed her. Resulting in her coming back as a BeastMan.
* TheMainCharactersDoEverything: While all of the characters have their own multiple interests, qualifying them as certain degrees of OmnidisciplinaryScientist, when it comes to their professional lives the show is actually very clear on what their specialties are. A long-form story arc has Leonard (experimental physics) devise a new idea of quantum mechanics, which he brings to Sheldon (theoretical physics) explicitly because he can't do the math that was needed. Howard (engineer) later suggests a gyroscope based on that idea, bringing all three together as Leonard and Howard work together on the mechanics of the device as Sheldon does the math needed to miniaturize it.
* MaleGaze: Howard used a remote controlled camera to check out Penny's cleavage while Penny and Leonard were finishing up their first date.
* ManChild: While all the characters indulge themselves in childish things, Sheldon insists that ''someone'' take care of him the way his mother would when he's sick (or even just ''home''sick), locks himself in his bedroom where no one else is allowed when he's furious, curls up into a crying ball of sad on his bed when he's embarrassed, runs away from home when he's upset, is practically traumatized by the sound of people arguing[[note]]Although this one may be perfectly justified, as his parents often got into arguments when he was a child and it is also implied that a lot of the arguments were exceedingly violent (he mentioned that his father would start throwing plates and shattering them in the kitchen, and that his mom planned to place glass shards in his father's meatloaf), even when they said to Sheldon that they stopped fighting[[/note]], and is stubborn and petty beyond all reason. The only thing on this planet that can force him to behave rationally when he's angry or depressed is an order from his mother, who Lenard calls "Sheldon's Kryptonite". He also responds positively to being patronized by Penny, especially if he gets a toy robot and a comic book out of it. [[LampshadeHanging Lampshaded]] by Bernadette at one point, after she successfully got him to go to bed by first reasoning with him about the effects of lack of sleep, and then simply treating him like an overgrown child and sending him to bed.
* ManicPixieDreamGirl: In its own way, this is what Penny is to all of the guys and especially Leonard. The guys are generally happy in their own closed-off world of science, sci-fi, and comics, and Penny encourages them to expand their social horizons a little. In a reversal, Leonard (and Sheldon to a similar degree) have introduced Penny into a different world of higher thinking and education. In "The Ornithophobia Diffusion" Leonard and Penny argued over how they hated the things they do together (Leonard hated doing karaoke at 2AM or going on hikes, Penny hated watching a documentary on dams), only to later use those same things when talking to other people about their "hobbies" to make themselves seem more interesting.
** Amy also seems to think Penny is this, ending up giving her [[UnwantedGiftPlot a gaudy painting]] of the two as appreciation for "transforming" her into a "hipster who wears a bra that clasps in the front."
** Howard's initial treatment of Bernadette would suggest he believed her to be this as well, when the truth is that she is often bringing him [[CloserToEarth down to Earth]]. In another reversal, Bernadette has fended off [[WhatDoesSheSeeInHim questions about why she is with him]] by suggesting that Howards' energy makes life a little more exciting.
* MasterOfTheMixedMessage: Before Leonard openly admitted his feelings to Penny (she was well aware of his crush beforehand) she had given him several mouth kisses, once [[AccidentalKiss when drunk]] and once as a "happy birthday" thing. But it also shows this trope depends largely on interpretation as Leonard went into a mental maze analyzing Penny's reaction to a brief relationship he had with Leslie. He wasn't certain if it was "Genuinely happy you're with someone" or "Too bad you're taken now" and (post breakup) "Genuinely sorry it didn't work out" or "Glad you're now available."
* {{Matricide}}: In "The Cooper Extraction", several WhatIf scenarios are depicted. In one of them an insane Howard preserves the mummified corpse of his mother. The cause of her death is not mentioned, but one of the interpretations mentioned by Bernadette is that Howard killed his own mother. Howard, the narrator of the scenario, does not care how his mother died as long as she is dead.
* MeaningfulName:
** [[DumbBlonde Penny]].
*** Interestingly enough, her name implies ''brightness'' -- i.e. "bright as a penny". This [[EpilepticTrees may be]] the creator's way of implying that she, as she has actually shown in many episodes, has more going on up there than many give her credit for.
*** [[LinkedListClueMethodology What's her name? Penny. Where is she from? Nebraska. What's the capital of Nebraska? Lincoln. Where's a common place to find Lincoln's picture?]] On a penny. [[EpilepticTrees We're through the looking glass, people.]]
*** And also, "''Penny'' for your thoughts?"
** The second syllable of Leonard's first name is...[[{{Nerd}} you guessed it.]] Leonard [[LampshadeHanging lampshades]] this in "The Adhesive Duck Deficiency".
** Howard Wolowitz's last name also sounds like a combination of "[[http://www.thefreedictionary.com/wallow wallow]]" and "wits." He wallows in his own wit, as it were -- or he would, if [[{{Jerkass}} Sheldon]] didn't constantly deflate him.
** The head of the Entomology Department at the university is Professor Crawley, he mentioned he had the nickname of "Creepy Crawley" in school.
* MeanwhileScene: In "The Stairway Implimentation", a flashback from Leonard contains one about what Sheldon does when Leonard, Raj and Howard left. Then lampshaded by Penny who notes Leonard couldn't have known what Sheldon did.
* {{Meganekko}}: Bernadette, Amy and Leslie.
* MetaCasting: Mayim Bialik, who plays Amy, actually has a [=PhD=] in Neuroscience (closely related to Amy's neurobiology). That makes her the only one in the cast who understands her dialogue. She even commented that the casting directors did a double-take when they saw that in her resume.
** After the disaster that was the original pilot, Lorre and Prady decided to change the female lead into a more adorable, sweet girl who would actually be appealing to the audience. They cast Kaley Cuoco because her natural personality was exactly what they were looking for (in addition to her [[Series/EightSimpleRules pre-existing credentials]]), in interviews the other members of the cast act significantly different from their characters except for Kaley.
* {{Metaphorgotten}}: Sheldon, referring to [[DreadfulMusician Penny's singing]].
--> '''Sheldon''': If cats could sing[[{{Beat}} ...]] they'd hate it too.
** Penny to Sheldon, while the former was extremely high on pain killers:
--> '''Penny:''' You know, people think you're this weird robot man who's so annoying all the time and you ''toootally'' are...but, then it's like that movie ''WesternAnimation/WallE'', at the end! You're so full of love, and you can save a plant, and get fat people out of the floaty chairs...
* MinorFlawMajorBreakup: Leslie breaks off her brief relationship with Leonard when she finds out that, in her ongoing debate with Sheldon over whether loop quantum or string theory is the superior model of spacetime, Leonard doesn't actually care, and would have let their hypothetical children "decide for themselves" which one they thought was correct.
* MinorInjuryOverreaction: Inverted, on a date with Penny at the gun range Leonard shot himself in the foot. It turned out that it merely grazed his foot but Leonard tried to play it off as being badass that he had a gunshot wound.
* MiseryPoker: Leonard, Raj, and Howard engage in this when Penny expresses disbelief that they haven't gone fishing with their fathers. Leonard and Raj's fathers may have been busy and scientific... but Howard's was ''missing''. Leonard even says "Okay, Howard wins."
* MistakenForGay:
** Leonard and Sheldon often introduce themselves as "living together" and causing people to initially jump to the wrong conclusion, done before the opening credits of the pilot episode. Raj's parents, after hearing Sheldon say he lived with Leonard, replied that they were "like Haroon and Tanvir." Leonard then intervened and explained the actual situation, although later as Sheldon ran to get their special-edition DVD of ''Fiddler on the Roof'' he remarked to himself that "maybe we ''are'' like Haroon and Tanvir."
** Sheldon also once went looking for an available male to distract Penny from an online gaming addiction. His technique was to approach a man in the cafeteria and ask if he was in a sexual relationship. Getting a no, he replied, "Would you like to be?" and walked away with the man's number scrawled on his hand.
** '''[[HeterosexualLifePartners Raj and Howard.]]''' In the third season, Leonard's mom encourages them to tell each other about their latent homosexual attraction, and tells Howard to stop talking about girls because it's upsetting to his "partner". Not to mention that Raj's parents say that Howard is the closest thing to a daughter-in-law that they have.
*** Leonard's mother calling Raj and Howard a homosexual couple is introduced in her first appearance on the show, the Season 2 episode "The Maternal Capacitance". Leonard loves the concept so much that he brings it up again in the above-mentioned third season episode, "The Maternal Congruence".
*** According to Raj's sister Priya his whole family thought he was gay
----> '''Priya''': My brother. He’s got a big crush on Bernadette.
---->'''Leonard''': What?! You’re kidding.
---->'''Priya''': No. I found poems he wrote about her. Very disturbing. “Oh, Bernadette, please play my clarinet.”... And for years, everyone in my family was convinced that HE was the '''clarinet''' enthusiast.
*** Lampshaded again in "The Cooper Extraction", an episode depicting WhatIf scenarios. Bernadette would never have ended up with Howard if not for Penny introducing them, because of Howard and Raj acting like a couple and Bernadette assuming they were.
* ModestyTowel: Penny calls out for help because she slipped and fell in the shower. She's able to get the attention of one of the male characters, who she convinces to come in despite the fact she took a shower (and presumably is naked). He walks into the bathroom, and despite that she can't move because she broke her shoulder, she's lying on the bottom of the tub completely covered by a towel. Possibly she grabbed it when she slipped and pulled it down.
* MomentKiller: Happens plenty of times between Leonard and Penny, and on occasion with Raj and Howard with their own prospective love interests.
* MoneyDearBoy: In Universe, the way Leonard and Sheldon got Prof. Proton to come to their apartment.
---->'''Leonard''': Sheldon wrote him a check.
---->'''Sheldon''': Big check.
** It should be pointed out that this is ''only'' in universe -- reportedly Bob Newhart was and is a big fan of the show, enjoyed his time there, and has made several more appearances as Prof Proton.
* MoodDissonance: Crops up here and there when the humor gets cringe-worthy enough, but the episode "The Staircase Implementation" deserves get a special mention. [[spoiler:Leonard is just about to ask Sheldon why he shoved him out of the elevator when there is a ''huge explosion'' inside it.]] Everybody except Sheldon looks absolutely horrified and the StudioAudience... keeps... on... ''laughing''...
** When Howard finds an Series/{{ALF}} doll, he reveals his mother gave him one when his father left and he pretended his father was taken to Alf's home planet. He asks the doll where his father was and is obviously just about to cry, yet the StudioAudience (or 'sweetened' LaughTrack) laughs through the entire scene.
* MoodSwinger: Part of the premise is that Penny is a very emotional girl who frequently baffles the guys with her illogical mood swings, shown in the pilot where she explains her life (from Nebraska, works at the Cheesecake Factory, etc) and then suddenly breaks down in tears (with Sheldon and Leonard looking at each other with a "Huh?" expression). At several instances she will storm into the apartment and yell at the guys for whatever recent mishap or screw-up, but she is also quick to forgive. Sheldon, being Sheldon, will sometimes associate it with "her time of the month."
* MoodWhiplash: Season 5 featured a visit from Sheldon's mother, Mary Cooper. Mary preferred to sightsee while Sheldon wanted her to do "mom things" for him. At the end of the episode, Sheldon gets sick and [[spoiler: his mother goes back to taking care of him. Sheldon sticks out his tongue at the group, knowing he's got his mom to care for him.]]
** Another Season 5 episode wraps up with [[spoiler: Leonard finding out Priya had cheated on him]] and while contemplating this, [[spoiler: Sheldon scares the crap out of him by jumping out of the couch.]]
** [[spoiler:When Howard finds out about his mother's death]]
** Sheldon's Meemaw finally comes to visit and he introduces her to Amy. She's the epitome of sweetness until he leaves the room to search her suitcase for a present. Then Meemaw lets Amy know she doesn't approve of her.
* MoralMyopia: Both in and [[RootingForTheEmpire out]] of universe Sheldon expects (and is expected to) the people around him to bend over backwards for him, but doesn't see any reason to do anything to reciprocate. Many times he considers the bare minimum that he does for others (such as asking how Leonard's day went while admitting that he doesn't care) is equal to the unreasonable demands he puts on his friends, because he doesn't see them as equals. For example he got into an EscalatingWar with Penny over the rules at his apartment but also sees nothing wrong with imposing his rules at her apartment or anyplace for that matter. In addition it is bad to give Sheldon Spoilers but it’s perfectly okay for him to give Spoilers to others.
** Sheldon also has no problem talking down to his peers or laughing at their mistakes yet gets incredibly depressed when the same thing happens to him.
** Penny was quick to point out that for a bunch of guys who claim they were bullied growing up, they showed absolutely no reservation in making fun of her friendly but not-particularly-bright friend Zack.
* MortalityPhobia: Sheldon plans to download his consciousness into a computer in order to live forever. When he gets concerned that the technology won't be available in his lifetime, he constructs a robot with a webcam and monitor so he can interact with others virtually while remaining sealed in his room away from anything that might harm him.
* MsFanservice: Penny. Her regular attire in the first two seasons was [[AbsoluteCleavage low cut]] tank tops and [[ShesGotLegs short shorts]]. Although it has gradually become averted as about mid-way through the second season she started showing less skin until the early third season where she hardly wears anything like that anymore unless going out to a party or dancing (although no less attractive).
** After Kaley Cuoco's real-life horse-riding accident they stopped showing her legs to hide her surgical scars, which had no in-universe explanation.
* {{Multiboobage}}: In "The Egg Salad Equivalency", Sheldon and Raj mention that Howard used University resources to build a six breasted sex robot.
-->'''Howard''': "Hey, I'm a feminist! I don't notice how many....breasts...a robot has...
* TheMultiverse: Frequently a DiscussedTrope.
--> '''Penny''': What's a Multiverse?
-->''' Sheldon''' : GET HER OUT OF HERE!
* MundaneMadeAwesome:
** The sequence when Sheldon and Raj try to solve the physics calculation on Sheldon's board: A series of still shots of them staring at the board, from different angles, set to "The Eye of the Tiger." Played for laughs, of course. With an aspirin break in the middle, during which the music stops. It resumes when the staring does.
** Pretty much ''every'' time the guys play video games together, you can expect them to be chewing the scenery into oblivion. "I possess the [[FeedMe SWORD OF AZAROTH!]]"
* MundaneUtility: It's a running gag for the characters using advanced scientific equipment for food-related purposes.
** Leslie Winkle used a large laser to heat up a cup of noodles, and flash-freezing and smashing a banana to put over her Cheerios.
** In "The Infestation Hypothesis" the guys use an expensive piece of lab equipment as a panini press.
** Raj and Howard use a 3D printer to make lifelike action figures of themselves, and Bernadette. For about a piece.
** Howard uses a robot hand designed for NASA to unbox their take-out food. It takes him a long time to program each movement, and they mention it took 28 minutes to do the entire bag. He later uses the robot hand [[ADateWithRosiePalms for more... intimate... tasks]].
* MushroomSamba: Howard gets some cookies from some old hippie women. They're laced with marijuana. He, Raj, and Leonard get high on the hash cookies and miss the meteor shower they went out to see.
* MustHaveCaffeine: Penny definitely seems to be this in the mornings. Drinking coffee while driving in the early morning is just one of the things she does that irritates Sheldon, and in the episode "The Vartabedian Conundrum", she comes into the apartment half-awake, saying desperately, ''"Out of coffee; need coffee."''
* MutualEnvy: Sheldon and Leonard, both brilliant scientists, envy each other their childhood. Sheldon's mum is too much of a religious nut for his taste (he still loves her), but he envies that Leonard's mother is rational, logical and a brilliant scientist herself. Leonard's mother is however extremely cold and emotionally abusive, and Leonard loves and envies how Sheldon's mother is sweet, caring and understanding.
* MyBelovedSmother:
** Howard's mother.
** Leonard's mother is an inversion; she is pretty much a female Sheldon. At one point, [[ParentalNeglect Leonard confesses to Penny that he built a hugging machine when he was a kid to use on himself.]] Even sadder, his dad borrowed it often.
* MyFriendsAndZoidberg:
** Leslie's usual approach to greeting the four guys.
---> '''Leslie''': Hey guys. (to Sheldon) Dumbass.
** When Sheldon's planning to move away, a section from his videotaped goodbye message to Leonard, Raj, Howard, and Penny: "The four of you are three of my closest friends, and one treasured acquaintance." The lowly "acquaintance" in question turns out to be [[spoiler: Howard Wolowitz]].
** The fact that Sheldon, Leonard and Raj are all doctors and Howard technically isn't is pointed out by several characters, including Dr. Gablehauser, who pointedly addresses him as "''Mister'' Wolowitz" and Sheldon, of course, who has referred to him as "Not-A-Doctor Howard Wolowitz."
---> '''Wolowitz''' (miffed): ''I have a masters' degree!''\\
'''Dr. Gablehauser''': Who doesn't?
*** When Sheldon gets a girlfriend who's a doctor and Wolowitz gets a girlfriend who's becoming a doctor.
----> '''Penny''': That means... Sheldon, you're a doctor. Leonard's a doctor. Amy's a doctor. Raj is a doctor. Bernadette is a doctor. And... Howard, you know a lot of doctors!
** Sheldon actually sees himself as this to Wolowitz. He refers to Raj as Wolowitz's best friend, Leonard as his secondary friend and himself as his tertiary friend.
** After Stan Lee's comic book signing:
---> '''Leonard:''' Look at that: 'To my friend Leonard, from Stan Lee. Excelsior.'\\
'''Howard:''' Awesome. Mine says, 'To my friend Howard, from Stan Lee. Excelsior.'\\
'''Raj:''' Mine says, 'To Raj, from Stan Lee.'
* MyGodWhatHaveIDone: Howard says this when he and Raj find the ''perfect'' match for Sheldon at the end of "The Lunar Excitation".
* MyHovercraftIsFullOfEels: Sheldon's attempts to learn Mandarin.
--> '''Sheldon''':
* MyRuleFuIsStrongerThanYours: After years of putting up with Sheldon's ridiculously elaborate roommate agreement, Leonard starts dating a lawyer, who knows how to exploit every loophole in the language of the contract.
** Played even straighter, when later Sheldon draws up a new roommate agreement that "benefits" himself greatly, then forces Leonard to sign it or expose them to Priya's parents.
* MySisterIsOffLimits: Raj invokes this when his sister comes to visit. [[spoiler:Not that it stops Leonard]]
** Sheldon also invokes this when his sister visits, as Leonard points out that she carries the ''potential'' to bear children as smart as Sheldon. [[spoiler: Not that is stops Raj and Howard from hitting on her in an extremely painful sequence:]]
---> Howard: "Did you know the Jewish people invented circumcision? You're welcome."

* NakedPeopleAreFunny: [[spoiler: Raj when he takes anti-anxiety pills that work a little ''too'' well.]]
** Professor Rothman, after his mental breakdown, often shows up at the University in the buff.
** On "The Parking Spot Escalation", Howard sits naked in Sheldon's spot, with Sheldon's laptop on his lap. [[spoiler:Later, in retaliation, Sheldon gets naked in Howard's new car.]]
* NamedAfterSomebodyFamous:
** The names Sheldon and Leonard are a ShoutOut to television writer [[http://en.wikipedia.org/wiki/Sheldon_Leonard Sheldon Leonard]].
** Barry Kripke is a ShoutOut to [[http://en.wikipedia.org/wiki/Saul_Kripke Saul Aaron Kripke]].
** Leonard's middle name is Leakey; his parents named him for [[http://en.wikipedia.org/wiki/Louis_Leakey archeologist and paleoanthropologist Louis Leakey]]. It's not related to his [[EmbarrassingMiddleName history of bedwetting]]. His last name, Hofstadter, is a ShoutOut to [[http://en.wikipedia.org/wiki/Robert_Hofstadter Nobel laureate Robert Hofstadter]] and his son [[http://en.wikipedia.org/wiki/Douglas_Hofstadter Douglas Hofstadter]] of ''Litearture/GodelEscherBachAnEternalGoldenBraid'' fame.
*** Possibly also to [[http://en.wikipedia.org/wiki/Richard_Hofstadter Richard Hofstadter]].
** Sheldon shares a last name with [[http://en.wikipedia.org/wiki/Leon_Cooper Nobel laureate Leon Cooper]].
** [[http://en.wikipedia.org/wiki/Raja Rajesh]] [[http://en.wikipedia.org/wiki/Ramayan Ramayan]] Koothrappali.
** Howard Wolowitz is "named after [[http://www.photobuff.com/filepro.htm a computer programmer]] known by the show's co-creator Bill Prady."
** Mary and Shelly Cooper have to be a shout out to Creator/MaryShelley, the writer of ''Literature/{{Frankenstein}}''.
* NeatFreak: Sheldon. Has a need to impose regimented order in his own apartment order, Penny's apartment, and even Howard and Bernadette's closet.
* NerdNanny: Penny. Serves as a caretaker to her nerdy friends.
* NerdsAreSexy: Pretty much the whole point of the show. The guys don't get laid despite their geekiness; they get laid ''because'' of it. (Except for the asexual Sheldon, for whom sex is less exciting than video games.)
** In fact, the tagline of the show's first season was "Smart is the new Sexy".
* NerdsAreVirgins: Played with. It's pointed out early on that everyone, even Raj (although Sheldon has no interest whatsoever), has had some intimate moments in the past. Their love life isn't exactly a feast though.
* NerdsSpeakKlingon: Practically a RunningGag.
** "The Bad Fish Paradigm": Sheldon asks Penny a few questions about her compatibility with Leonard and one of them is "Do you speak Klingon?"
** The episode, "The Panty Pinata Polarization" starts with the guys playing a game of Klingon Boggle.
--->'''Sheldon''': Excuse me, Penny, but we’re…\\
'''Leonard''': No, no, don’t tell her.\\
'''Sheldon''': …playing Klingon Boggle.\\
'''Leonard''': Aw!\\
'''Howard'''': What do you mean, aw? Like she didn’t know we were nerds?
** "The Creepy Candy Coating Corollary": Howard says he speaks six languages if you count Klingon, Leonard and Penny agree that "Girls don't count Klingon". Later, after Sheldon wins a game in a Mystic Warlords of Ka'a tournament, he shouts at Creator/WilWheaton in Klingon.
--->'''Wil''': Did that guy just say 'Revenge is a dish {{best served cold}}' in Klingon?\\
'''Stuart''': I believe so.\\
'''Wil''': What is wrong with him?\\
'''Stuart''': Everyone has a different theory.
** "The Vacation Solution" Penny thinks that Bernadette has talked Howard out of having their wedding invitations in Klingon, [[spoiler: but Bernadette tells her that the Klingon text is on the back.]]
** "The Hawking Excitation": Howard tells Sheldon no in several languages, including Klingon and Binary coded Ascii.
** "The Countdown Reflection": Sheldon keeps trying to preform his part of the wedding ceremony in Klingon.
--->'''Bernadette''': Sheldon! I told you no Klingon!
** In "The Romance Resonance", Howard sings to Bernadette and on of the things he lists that he would do without her is "speak a lot more Klingon".
** "The Proton Transmogrification": While attending a funeral, Penny and Leonard both get pensive about dying and any regrets they might have.
--->'''Penny''': What would you regret?\\
'''Leonard''': Mm, you know, that I didn't travel more, take more risks, learn another language.\\
'''Penny''': You know Klingon.\\
'''Leonard''': That's true.\\
'''Penny''': No, I meant that as a regret.
* NeverLendToAFriend: Sheldon honestly doesn't care about the money he lends Penny, but ''she'' gets neurotic about it, misinterpreting Sheldon's every innocent comment as a snide remark about her debt.
* NeverMyFault: When Sheldon receives a ticket for running a red light - issued by a traffic camera - he immediately blames Penny.
* NewPowersAsThePlotDemands: Howard was never seen demonstrating his ability to communicate in American Sign Language until Season 5's "The Wiggly Finger Catalyst," although several times (including the pilot) he likes to make himself out as being a linguist (including knowing Klingon). ItMakesSenseInContext because Howard acts as an interpreter between Raj and Raj's deaf girlfriend Emily in the episode, and as an in-joke that even with a woman Raj actually can date comfortably, [[HeterosexualLifePartners Howard still has to be there]].
* NewMediaAreEvil: Averted notably in-universe whenever someone appears on National Public Radio: no one listens on an actual ''radio,'' but appear to listen to the live stream.
* NiceGuy:
** Leonard Hofstadter.
** Stuart.
** Zack Johnson, one of Penny's exes. He is [[TheDitz conspicuously thick]] but in refreshing contrast to [[JerkJock the rest of Penny's exes]] doesn't seem to have a bad bone in his body.
** Creator/JamesEarlJones is definitely this, though he has a bit of a prankster side (as Creator/CarrieFisher knows).
* NiceJobBreakingItHero: Penny has a bad break up with an unseen boyfriend at the end of Season 1, and Leonard decideds to go be a comforting friend. As he tries to make her feel better about the situation, he unwittingly convinces her that she should give the guy another chance. In a double layering of the trope, Penny's attempts to reconcile with the boyfriend fall apart (due to him already in bed with another girl) and she [[MoodSwinger gets angry]] at Leonard for his "advice," leading Raj to say, "Wow! You managed to screw-up the screw-up!"
* NiceJobFixingItVillain: Priya cheating on Leonard in a Season 5 episode. If she didn't do that, Leonard and Penny would not have reunited later on.
* NightmareFuel: Some disturbing stuff are acknowledged in-universe and they indeed invoked nightmares.
** Leonard has a nightmare about Sheldon's reproduction and the mitosis method.
*** Also, Goofy. [[MakesJustAsMuchSenseInContext Leonard doesn't understand that one, either]].
** Sheldon himself has nightmares about [[Film/TheTimeMachine1960 Morlocks]] and [[Literature/TheLordOfTheRings Gollum]], specifically [[spoiler:himself turning ''into'' Gollum.]]
** When Bernadette describes a real lab accident and then lies about it, the others look disturbed. As they should be.
** Santa Claus fuels Sheldon's nightmares.
* NoEnding: Several episodes don't really resolve the story, and the next episode just carries on without acknowledging it.
** "The Twenty-One Second Excitation" almost has a BolivianArmyEnding. In the final scene before TheTag, [[spoiler:Sheldon and the guys make off with a print of ''Film/RaidersOfTheLostArk'', a horde of other nerds (including Evil Creator/WilWheaton and Captain Sweatpants) [[TorchesAndPitchforks hot on their heels]] as the movie’s theme music plays.]] How is this resolved? Did they get away? Did they get trampled? Did Sheldon wind up in the hospital, or jail, or the loony bin? The Tag just cuts back to Penny, Bernadette, and Amy Farrah Fowler in Penny’s apartment. The episode ends there, and the plot is never revisited.
** In the second season premiere, Leonard accidentally insulted Penny when discussing their relationship and the episode ended with her closing the door in his face. It was some 5 episodes later when we even learn that they had an uneasy agreement to stay friends and ignore their attempts at a romantic relationship, but the exact details of which are never seen.
* NoGuyWantsAnAmazon: S4 Ep05, "The Desperation Emanation", Bernadette sets Leonard up with her tough friend, Joy. Leonard ends up being miserable on their date due to her overbearing and shrill personality, including constant references to how strong she is and how much she's into fitness and martial arts (including 101 ways to rip a [[GroinAttack guy's nuts off]]). She also makes unwelcome comments about her bathroom habits. Although, when she implied she would put out he still agreed to be her date to a wedding.
* NoIndoorVoice: Mrs. Wolowitz.
* NoManShouldHaveThisPower: Parodied in "The Precious Fragmentation", where Leonard announces that he's mailed the prop One Ring back to Peter Jackson where it belongs. [[spoiler:[[SubvertedTrope He's lying.]]]]
* NoOSHACompliance: In "The Engagement Reaction" there is a patient in the hospital being treated with a very contagious disease. As it so happens the door doesn't have a latch and swings open freely, allowing Sheldon (who is desperate to avoid any sick people) to accidentally walk in without seeing the quarantine sign.
* NoSenseOfHumor: Subverted by Sheldon; he's got one, it's just... very Sheldon.
* NonSequitur: A Non Sequitur question. In Amy's ''Series/LittleHouseOnThePrairie'' FanFic, Amelia loves Cooper but has trouble saying it. Leading to this: "All she longed to do was ask if his heart was beating as fast as hers, but she was too afraid to hear the answer. So instead she asked him in the future, if Montana would ever become a state...". This comes out of nowhere and is irrelevant to the story.
* NoodleIncident:
** Sheldon's childhood seems to be filled with these. Among the ones mentioned are Sheldon's dad fighting a bobcat over some liquorice, his parents arguing as his father trap shoots the family's commemorative plates off the roof, his mom's secret smoking habit, trying to build a nuclear reactor in his back yard, trying to frighten neighborhood bullies with a homemade DeathRay, being chased up a tree by a neighbor's chicken, getting beaten up by his sister, and losing an uncle who died chasing a badger up a chimney. Assuming Sheldon really CannotTellALie, it's a miracle he didn't turn out even more neurotic.
** Amy's 'firm cervix' and her 'tilted uterus' was supposed to be next.
** Penny was quoting her dad on how much he likes Leonard by comparing him to her rather peculiar collection of old boyfriends "Why can't you go out with Leonard? Leonard never tipped a cow on himself."
** And the winner:
---> '''Howard:''' Close your eyes. Put out your hand. I got you something special.
---> '''Bernadette:''' Come on, Howard, I'm not falling for that again.
** How Sheldon got restraining orders filed against him by Leonard Nimoy and Carl Sagan.
** Raj mentions an incident involving Sheldon getting food poisoning at the Rose Parade.
** Howard tricks Penny into opening the door by using a voice recording to impersonate Sheldon. Penny mentions she wouldn't open the door for Howard after he gave her a teddy bear with a ''webcam'' hidden in it.
* NotDistractedByTheSexy: Invoked by Penny for laughs on two occasions -- one time, when the guys are discussing possible improvements to her entertainment system she makes the offhand comment that she would take all her clothes off because it's hot, and another time she brings three of her girlfriends to their apartment and offers to have sex with them while they are playing ''Halo 3''. They don't even hear her.
** Though, to be honest, what are the odds the girls would have made good on the offer if the guys had gone for it? Might as well carry on enjoying what they are doing.
* NotEvenBotheringWithAnExcuse:
** One episode had Sheldon anxious at the prospect of giving a speech at an awards banquet. His friends offer to help him work through his anxiety in different ways. Leonard offers to do a therapy session with him, Raj says he will teach Sheldon some Indian meditation techniques, and Penny agrees to take him to buy a new suit for the occasion. Sheldon asks what contribution Howard will make, and Howard replies, "My contribution is to pretend I give a damn about your piddly-ass problems. And that's 24 hours a day, buddy." Sheldon appreciates his honesty.
** When Raj has his date with Lalita, Sheldon excuses himself for being late, when asked what happened he just admits that he didn't want to be there.
** Similarly, in "The Zazzy Substitution," Penny gets out of an awkward conversation at the Cheesecake Factory by saying, "Okay, [[ScrewThisImOuttaHere I'm just gonna walk away]], 'cause I don't want to be here."
** In "The Lunar Excitation," Leonard asks Sheldon to move the nitrogen tank. Sheldon says he can't and when Leonard asks why, he says, "It's very heavy, and I don't want to."
* NotHelpingYourCase: Amy and Sheldon accused Leonard of being a sore loser after he "lost" an attempt at Counterfactuals. The game ''is'' very hard to win (unless you think like Sheldon and Amy), but Leonard's response didn't exactly do him any good...
-->'''Sheldon''': You should have been here for the Great Jenga Tantrum of 2008.
-->'''Leonard''': You bumped the table, and YOU KNOW IT!
* NotSoDifferent: Amy ended up looking through Leonard's yearbook and based on the comments noticed how unpopular he must have been, only to reveal that no one even ''signed'' her yearbook except her mother. This actually ended up as an ice breaker between them, who didn't really talk much before then.
** Penny calls out the guys for making fun of Zack for not knowing much science in "The Justice League Recombination" when they were always bullied.
** In "The Dead Hooker Juxtaposition", when a new neighbor threatens to replaced Penny as the new DungeonMastersGirlfriend, she complains that she's just manipulating the gang to do things for her, such as sending Howard to retrieve a TV from a relative. Sheldon notes that ''Penny'' did the exact same thing when they first met, sending Leonard and him to retrieve her TV from her ex-boyfriend.
---> '''Penny''': Apples and oranges, Sheldon!
** In "Higgs Boson Observation" Amy dragged Penny to the university to spy on Sheldon to make sure there isn't anything going on with his new (female) assistant Alex. While in his office Amy started licking his pencils and rubbing his stapler on her body to "[[CloudCuckoolander spread her scent]]" and effectively mark her territory, which Penny expressed exasperation over. They eventually find that Amy shouldn't worry over Sheldon being attracted to Alex but do find Alex and Leonard having a friendly chat, which incites some jealousy in Penny. When Alex visited the guys apartment Penny acted far more possessive of Leonard than normal and eventually took him to bed, in effect doing the same basic "marking her territory" thing that Amy was doing.
** Used yet again when Raj tagged along for girls night out. He mentioned offhand about how his love life hadn't been going well because he had a habit of crushing on the girlfriends of his friends like Penny and Bernadette, [[InnocentlyInsensitive leaving Amy out of the loop]]. While hurt, she admitted that before meeting the group she knew what it's like to be left out and the two ended up bonding over the fact they were/are constantly rejected. By the end, Raj mused that she was suddenly [[{{Irony}} much more attractive to him]].
** In "The Scavenger Vortex" Howard and Amy get teamed up for Raj's scavenger hunt and bond over their mutual love of Neil Diamond. It goes so far as the two of them abandoning the hunt to sing karaoke.
* NotThatKindOfDoctor:
** In reference to Leonard's new girlfriend, an actual ''medical'' doctor.
--->'''Penny''': So who is she?\\
'''Leonard''': She's a doctor.\\
'''Penny''': Nice. A doctor doctor, or a you kind of doctor?
** Also happens when Sheldon calls the police to help with his hacked ''VideoGame/WorldOfWarcraft'' account, from "The Zarnecki Incursion":
--->'''Police Officer:''' I'm sorry but I can't help you, Mr. Cooper--\\
'''Sheldon:''' ''[[InsistentTerminology Doctor]]'' Cooper.\\
'''Police Officer''' (looks over at Leonard incredulously)''':''' ...Seriously?\\
'''Leonard:''' Not the kind with access to drugs.
* NWordPrivileges: Invoked. Raj claims that when ''he'' refers to India as "Gandhiville", it's not racist.
** Raj also calls out Penny for calling Jar Jar Binks stupid:
-->''We'' can say it, not you!"

* ObjectCeilingCling: Howard's faulty space toilet malfunctions, and during their attempt to fix it, he launches a piece of meatloaf onto the apartment ceiling. It falls down some time later when Penny is there.
* OddNameOut: Given in-universe to one of his "clowder or glaring" of cats by Sheldon. While he named most of them after scientists in the Manhattan Project like Dr. Robert Oppenheimer, one was named Zazzles because, according to Sheldon, "he's just so zazzy".
* OffScreenBreakup: Howard and Bernadette. They got back together, though.
* OffTheShelfFX: In "The Cooper/Kripke Inversion", Howard scans Raj for his action figure with an UsefulNotes/XBox360 Kinect sensor.
* OfficialCouple: Leonard and Penny. It was semi-official as of the pilot episode, and they were an actual couple for most of Season 3.
** Sheldon and Amy.
** Howard and Bernadette.
* OhCrap: Penny in "The Roommate Transmogrification" when she [[spoiler: wakes up and discovers that she has had sex with Raj.]]
** Sheldon when he realizes that he [[spoiler: told an FBI agent that Howard crashed the Mars Rover.]]
** Sheldon again when he realizes Amy's plan to get him to associate happy memories with her presence is actually working.
* OmnidisciplinaryScientist: Sheldon, a string theorist who on his days off also dabbles in, for example, genetic engineering, and who purportedly built [[MadScientist a nuclear reactor and a "death ray"]] as a [[TeenGenius child]], not to mention his extensive knowledge on the Social Sciences (History, Political Science, and Flags). This [[DependingOnTheWriter Depends On The Writer]], though: we've also seen him unable to ascertain how to open a toolbox. Mainly, he just ''thinks'' he's an expert on everything; when he spends a day in Amy's lab, he thinks his intelligence qualifies him to do real neurobiology work, but can't correctly perform even the simplest tasks.
** TruthInTelevision -- anyone who's ever worked with extremely high-IQ academics will marvel at their often-paired lack of common sense and/or inability to perform mundane tasks. This is reflected in the reality that lower-level degrees are more general studies while higher level degrees tend to focus on specific specialties; thus two people could both have a doctorate in, say, electrical engineering and have little in common because one is for power-plants and the other is for microprocessors. Mayim Bialik is shown to be very particular in her phrasing when discussing Amy's neurobiology degree versus her own neuroscience degree because they ''are not'' the same thing.
** This trope is generally subverted on other occasions, Howard (the engineer) boasts that he designs items for the international space station but he has no idea how to fix the elevator and when their car breaks down Leonard has this to ask:
--->'''Leonard''': Anyone here know how an internal combustion engine works?\\
'''Raj, Howard and Sheldon''': Of course!\\
'''Leonard''': Any know how to ''fix'' an internal combustion engine?\\
'''Raj, Howard and Sheldon''': Not a clue.
* OnceASeason: A couple of recurring gags you can expect every season: "Soft Kitty" being sung to or by Sheldon, Leonard and Sheldon's parents visit (although somewhat sporadically), everyone appears in costume for some event (Halloween party, Renaissance fair, the punishment for losing a game...) and at least one appearance by Creator/WilWheaton (from season three onwards) and Creator/StephenHawking (season five onwards).
** Each season also has at least one Leonard/Sheldon blow-out that only increases in intensity every year. The first season had an argument over the physics bowl, second season was Sheldon's interference with Leonard's relationship with Stephanie, the third season led to a WholeEpisodeFlashback with Leonard relating to Penny how they became roommates, the fourth season Priya started poking holes in the roommate agreement, the fifth season the roommate agreement was temporarily nullified and the sixth season Leonard started making plans to move in with Penny.
* OneHeadTaller: Penny is generally about the same height as Leonard (whose short stature is frequently commented upon) but when she wears heels she suddenly towers over him.
** Sheldon, being there is six inches between him and second tallest member of there group (and fifteen inches between him and the shortest) he towers over all the others. His [[StatuesqueStunner twin sister Missy]] shares this physical trait with him, towering over all of the guys except her brother.
* OnePersonBirthdayParty: In "The Cooper Extraction", several WhatIf scenarios are depucted. In one of them, Amy remains friendless and celebrates her birthday alone. She tearfuly sings the "Happy Birthday" song to herself:
-->Happy birthday to me
-->Happy birthday to me
-->There are tears in the frosting
-->Happy birthday to me.
* OneSteveLimit: Averted. There are two Angelas. One is Angela, a love interest for Raj, and the other is Angela Page, an FBIAgent. Both appear in Season 4.
** In similar sounding names, there is Alicia the actress in Season 2 and Alice the comic book artist in Season 5.
** Same name, slightly different spelling. There is Dmitri, a former physicist and current janitor in Season 1, and Dimitri Rezinov, a Russian cosmonaut in Seasons 5 and 6.
** Raj has dated two girls named Emily. One was a deaf girl in Season 5. The other is Dr. Emily Sweeney in Seasons 7 and 8.
** There are three Erics so far. One is Dr. Eric Gablehauser, the head of the Physics Department in Seasons 1 and 2. The second is Eric, who briefly dates Penny in Season 2. And the third is is Eric, a man who befriends Sheldon in Season 7, since they both love trains.
** There have been four Georges so far. The first is George Cooper Sr., Sheldon's deceased father. The second is George Cooper Jr., Sheldon's brother. Both are unseen characters. Then there are two guest stars playing fictionalized versions of themselves: Nobel laureate Dr. George Smoot in Season 2 and actor Creator/GeorgeTakei in Season 4.
** Same name, different spelling. There is Glen, a mall security guard in Season 3, and Glenn, Bernadette's former college professor and boyfriend in Season 4.
** In similar sounding names, there are three characters who sound alike. One is Joey Rostenkowski, Bernadette's brother. An unseen character. Second is Joy, a fitness fanatic dating Leonard in Season 4. The third is Joyce Kim, a North Korean spy who dated Leonard back in 2003.
** Another set of similar sounding characters. One is Katie, the female lead of the Unaired Pilot. The other is an actress playing a fictionalized version of herself in Seasons 3 and 4, Creator/KateeSackhoff.
** Another set of similar named characters, five of them. There is Michael Hofstadter, Leonard's brother. An unseen character. Then there is Mikayla, a Las Vegas prostitute in Season 2. Then a trio of Mikes. One is Mike, who briefly dated Penny in Season 1. Second, there is Mike Massimino, Howard's NASA colleague in Seasons 5-8. Third, there is Mike Rostenkowski[, Bernadette's father. He appears in Seasons 5-7.
** There are two Sanjays. One is Sanjay, a former boyfriend of Priya. The other is Sanjay, a cousin of Raj and Priya. Both are unseen characters.
* OnlyChildSyndrome: Very much averted. Howard is the only main character in the series who is explicitly stated to be an only child. Sheldon has an older brother, George, and a twin sister, Missy; Leonard has an unnamed older sister and a younger brother, Michael; Penny has an unnamed brother and sister; and Raj has a younger sister, Priya, three unnamed brothers, and another unnamed sister. Bernadette, Amy, and Stuart have an unspecified number of siblings as well. Numerous aunts, uncles, cousins, nieces, nephews, and grandparents are also mentioned throughout the series.
* OnlyOneName: Penny's last name has not been revealed in the show. This was revealed by the writers to be something of an accident, since the guys are (mostly) [=PhD's=] they were referred to as Dr. Cooper, Dr. Hofstader, etc. and their diplomas' are seen on the wall. With Penny they didn't even realize they hadn't revealed her last name yet, and by the third season they felt supersticious about doing so. Of course, by the ninth season she does have a known last name... [[TheyDo Hofstader]].
* OnlySaneMan: Mostly Leonard. Since his fixation on/relationship with Penny [[ShippingBedDeath drifted away from center stage]], this has become a big part of his role.
-->'''Sheldon:''' I'm afraid I can't allow that. Pursuant to Starfleet General Order 104, Section A, you are deemed unfit and I am relieving you of command.
-->'''Leonard:''' Starfleet General Order 104, Section A does not apply in this situation.
-->'''Sheldon:''' Give me one good reason why not.
-->'''Leonard:''' Because THIS IS NOT ''STAR TREK!''
** Penny possibly counts too, especially compared to Amy and maybe Bernadette.
** Subverted in "The Love Car Displacement" during the science panel, when ''Sheldon'' of all people is (briefly) the OnlySaneMan.
* OnTheRebound:
** Leonard's attempt at a "serious" relationship with Leslie was this, after his first time attempting to date Penny went south. At one point he and Penny run into each other across and the hall with their new romantic partners. They swoop their respective into a passionate kiss and try not to stop until the other does (which goes on for at least a few minutes) in an effort to show just how "fine" they're doing without each other.
** After Leonard and Penny tried dating the second time Penny gets together with Zach. She admits that while he's a nice guy, he's dumb as a sack of hammers and the only reason she bothered to give him the time of day was so she wouldn't be lonely and to take her mind off of Leonard.
* OperatorFromIndia: InvokedTrope, When the gang starts up a software development company Raj is offended that Sheldon's assigned him a job in customer service; Sheldon explains that it's in order to give the impression that they're a large multinational.
-->'''Raj:''' "Ohhh, very clever. But still racist."
** In another episode, Raj suggests that he would [[BriefAccentImitation put on an American accent]] when dealing with tech support because he doesn't want them thinking he was making fun of them.
* OrderedApology: Often, and Sheldon is usually involved. Sheldon's mother tends to be brought in to force him into apologizing, and Missy deals with his unapologetic stubbornness through GroinAttack quite effectively as well.
* OrphanedPunchline: "So anyway, to make a long story short, turns out I have an unusually firm cervix"
* OutdatedOutfit: Leonard's collection of ugly suits, including a felt maroon-ish one. Sheldon also has a suit more appropriate for the 1930's.
* OutGambitted: Sheldon tries the old "joy-buzzer" prank on Howard by getting Howard to shake Sheldon's hand with the buzzer attached. Howard collapses and Bernadette is immediately upset, reminding Sheldon that Howard has a heart condition. Bernadette pulls out an adrenaline needle and insists that Sheldon inject it into Howard's heart, claiming not to be strong enough to do so herself. Howard opens his eyes and reveals the ruse. This one seems like it had to have taken some planning and anticipation on Howard and Bernadette's part for whatever Sheldon had in store, but it may have been easier if you factor in Howard's familiarity with The Sheldon as demonstrated in the above ImprobablyPredictable example.
** It gets even better when Sheldon smacks himself in the forehead for falling for it and gets a dose of his own joy-buzzer.
** In the same episode, Sheldon apparently rigged a prank in the mail box and was very obviously trying to lead Leonard to it. Leonard figured out the prank beforehand, and rigged it so it was on remote control and would go off when Sheldon opened the box and was greeted with a blowup pillow with Leonard's face on it. This pretty much sums it up
---> '''Leonard:''' You may be from Texas but I'M from Jersey
* OutOfCharacterMoment: Happens to Penny in the episode "The Dead Hooker Juxtaposition". When Alicia (a new, pretty girl) moved upstairs and was manipulating the guys with her looks, Penny had a conversation with her in the laundry room. She tried to explain the idea that they don't know how to put their shields up...
--> '''Penny:''' You know, in ''Franchise/StarTrek'' when they go into battle they put their shields up... ''(realizes what she just said)'' ...Where the hell did THAT come from?
** Happens to Sheldon in the episode "The Adhesive Duck Deficiency". This fuels the comedy when Sheldon is the only one to help Penny when she dislocates her shoulder in the shower. Penny asked him to try to be "comforting" and Sheldon's attempt to be comforting is so alien that Penny is visibly disturbed by it.
** Happens to Howard in the episode "The Financial Permeability". He and the other guys attempt to confront Kurt about the money he owes to Penny. Howard is afraid of what might Kurt do to them. So he makes the sign of the cross for protection. But Howard is not a Christian but a [[JewishAndNerdy Jew]].
* OvercomplicatedMenuOrder: A RunningGag, with Sheldon being ''very'' specific on how he wants his food prepared. It was also done once with Leonard's mother.
** In the episode "The Einstein Approximation", where Sheldon decides to work at the Cheesecake Factory in order to let his mind work on a physics problem, Leonard gives him an extra-complicated order in revenge for all the times Sheldon did it to him.
* OverprotectiveDad: Intentionally subverted/invoked with Penny's father, who really likes Leonard and considers him far above the dumb, brawny jocks she usually dates. When he discovered they actually didn't [[TheBeard get back together]], [[spoiler: he implies he's going to verbally assault Leonard, only to quietly beg him to not give up on her. He then tries to use reverse psychology on Penny (who was in her bedroom) by yelling at Leonard to never see her again.]]