* ActorSharedBackground: Raj was originally going to be an American-born Indian until Nayyar was cast, with him being much more Indian (actually raised in India) than planned. JimParsons is also from Texas like Sheldon is and although Parsons' accent doesn't come in very often with Sheldon, Sheldon's mother and sister have very thick drawls. Mayim Bialik, who plays neurobiologist Amy Farrah Fowler, has a [=PhD=] in neuroscience.
* AdoredByTheNetwork:
** In Australia, the Creator/NineNetwork has this feeling toward the series; it usurped ''TwoAndAHalfMen'' as their go-to filler whenever there's a gap in the timetable, and between the main Nine channel and its spillover GO!, it's shown with incredible frequency.
** It's in constant looping on the UK's "E4" channel as well. Not just one loop either, but two. The loops are deliberately out of synch so that one might be in season 3, while the other is still in season 1.
** In the U.S. it's replaced ''Series/TheOfficeUS'' as TBS's Tuesday night marathon show.
* CreatorThumbprint: In "The Gothowitz [...]", Leonard, Sheldon, and a confused Penny are watching the fictional anime ''"Ochikurru: Demon Samurai"''. In series creator ChuckLorre's other most famous show, ''TwoAndAHalfMen'', Charlie was once hired to write the theme song for the show when it was being developed.
* DuelingShows: With ''Series/{{Community}}'', due to the focus on geekdom.
* FakeNationality: Averted with Raj but not with his parents. British-Israeli Brian George and British actress Alice Amter play Indians. Raj's sister Priya (Aarti Mann) was born and raised in Pittsburgh.
* [[HeyItsThatGuy Hey, It's That Girl/Guy!]]: An in-universe example, when SummerGlau appears (as herself) in the episode "The Terminator Decoupling"
** Sheldon's old room mate is [[Series/TheWalkingDead Glenn]]
** Also, c'mon, how many people freaked out when they realized "hey, that's [[DrHorriblesSingAlongBlog Moist]]!"
*** Or "hey, that's [[{{Roseanne}} David!]]
**** What about, "Hey! That's [[{{Roseanne}} Darlene!]]"
***** Or "Hey! That's aunt [[{{Roseanne}} Jackie]]!"
** The aggravated insect specialist is Lewis Black from ''Series/TheDailyShow''.
** Hey, it's the [[Series/HowIMetYourMother pineapple incident]] girl !
** Hey, that's {{Blossom}}!
*** Oddly, in an earlier episode, Raj had mentioned the actual actress Mayim Bialik. So in this universe, Amy Farah Fowler (played by Bialik) ''and'' Bialik herself both exist.
*** Even stranger, Johnny Galecki had a very brief appearance on {{Blossom}}--he played Blossom's date.
**** Possibly ActorAllusion in one episode where Galecki's character Leonard then went as Amy's date to a wedding.
** Hey, that's Bridget from ''Series/EightSimpleRules''!
** Hey, Its [[WesternAnimation/TheSimpsons Lisa Simpson]] helping Sheldon find a new vocation in "The Einstein Approximation"!
** [[Creator/WilWheaton Wheeeeeeeeeeeatooooooon!]]
** Hey, Penny's ex is [[TheNeidermeyer Encino Man]] from the ''GenerationKill'' HBO MiniSeries
** Hey, it's Christine Baranski, who played an {{Expy}} of [[Series/AbsolutelyFabulous Patsy Stone]] on ''Cybill.'' As Leonard's mom.
** Hey, it's [[LessThanPerfect Sara Rue]].
*** In one of the same episodes she appeared in, Penny is trying to make Leonard jealous by kissing on [[Series/{{Scrubs}} Keith Dudemeister]]
** Ohhh, [[{{Dollhouse}} Echo's been imprinted as an FBI Agent again]]!
** Hey, Penny's dad is [[Series/{{Dexter}} Agent Lundy]]! And he still wants a son despite having had FIVE sons in CompleteSavages.
** Hey, Leonard is [[NationalLampoonsChristmasVacation Rusty]] all grown up!
** Didn't Bernadette briefly date [[Series/TrueBlood Hoyt Fortenberry]]?
** Hey, Raj and [[Series/StarTrekDeepSpaceNine Julian Bashir]] have the same father!
** Hey, Dave Underhill used to be used to be [[Series/BattlestarGalacticaReimagined an ace Pyramid player!]]
** Hey, it's Billie from ''Series/{{Charmed}}''!
** Howard's mom sleeps with [[Series/BuffyTheVampireSlayer principal Flutie]]!
** Hey, the cute deaf redhead that Raj dates in one episode is [[Series/SwitchedAtBirth Daphne Vasquez]]!
** Hey, Sheldon's sweet mom is that psychotic woman from DesperateHousewives [[http://www.youtube.com/watch?v=PXDK5aNCQUo]]!
* HeyItsThatVoice: In "The Einstein Approximation", Creator/MarkHamill's DCAU Joker laugh can be heard as Leonard's ringtone.
** In the same episode, [[WesternAnimation/TheSimpsons Lisa]].
* LifeImitatesArt: [[JohnnyGalecki Leonard]] and [[KaleyCuoco Penny]] actually did date for two years; they did so in secret and only revealed it after they had seperated.
* OneOfUs: Nobel Laureate George Smoot made an appearance on the show after expressing his love of the show to the producers, with the episode actually written for him.
** Melissa Rauch said she was a fan of the show before she was cast as Bernadette. On the other hand, Mayim Bialik admitted she didn't watch the show before she auditioned.
* RealLifeRelative: In the 2013 Valentines Day episode Kaley Cuoco's sister Briana played Gretchen whom an ex-boyfriend of Penny's cheated on her with.
* RomanceOnTheSet: Johnny Galecki and Kaley Cuoco--yes, Leonard and Penny. However, they kept it secret until after they broke up... which was around the time their characters first got together. (They kept it secret because of the work environment and remain friends.)
* ThrowItIn: A noted aversion. The cast has said that they know almost as much about the nerd culture references as they do the {{technobabble}}. Going off script would be suicide.
** Melissa Rauch said that her vocal pitch for Bernadette was based on her mother's voice and thought it would fit the character, though she decided not to push it by including a New Jersey accent.
** In "The Boyfriend Complexity" Sheldon angrily tosses a large stack of loose papers in the air in frustration over hard work that was ultimately pointless. As the pages floated to the ground one managed to perch ''perfectly'' on his shoulder, and Parsons managed to stay in character long enough for that shot to remain in the final cut (trying to stare in anger at Galecki, he wasn't able to say his next lines without cracking up).
** The, somewhat infamous, spanking scene with Sheldon and Amy was not originally going to be on camera. The episode was supposed to have a RunningGag of hearing Sheldon taking care of Amy and having some InnocentInnuendo implied coming from the bathroom as the audience listens from the main room. As filming for the episode began the joke wasn't working the way they wanted it to and so they ended up actually showing the scene in front of the audience. The other cast members commented that they would emerge from their dressing rooms and were shocked.
* WhatCouldHaveBeen: The original (unaired) pilot for the series was quite different; Sheldon and Leonard were the only carry-overs. The Penny role was a girl named Katie (played by Amanda Walsh), a {{hard drinking party girl}} who was a new roommate and the guys would help her break out of her shell. Leonard and Sheldon's only friend was a female co-worker named Gilda (Iris Bahr) who had a crush on Leonard, felt threatened by Katie's presence, and had once slept with Sheldon (who wasn't asexual). Lastly the set design was much darker with them living in a ''very'' run-down building. Test audiences ''hated'' Katie but really liked Leonard and Sheldon, which gave them a rare opportunity to revise the concept with a second pilot. Many of the same jokes and lines of dialogue were reused in the first season, especially the introduction to the guys white boards and their argument over the number of dimensions used.
** Kaley Cuoco was considered for the role of Katie, but was busy with other things at the time and it was felt the cynical character wasn't a good fit for her. When creating Penny they actually had Kaley in mind to reflect a much more bubbly and warm individual, to avoid the trappings of the Katie disaster.
** Bill Prady said that Kevin Sussman (who portrays Stuart) was a runner-up for the role of Howard.
** The show originated under two different concepts, one being Bill Prady's experience working with a group of eccentric computer programmers back in the 80's and another under the idea of a young woman entering the real world of finding a job and making new friends. Realizing the key to both shows was on the secondary cast providing some contrast they decided on their own XMeetsY and combined them. After that they decided to move the genius nerds away from computers and make them scientists instead, pushing them outside a comfort zone.