[[quoteright:240:[[VisualPun http://static.tvtropes.org/pmwiki/pub/images/weasel_55.jpg]]]]
[[caption-width-right:240:This is not a Necessary Weasel. It's a Gratuitous {{Weasel|Mascot}}, courtesy of the RuleOfCute.]]

->''"Be happy that weasels infest the world. Weasels are like motor oil for society. It wouldn't be fair to judge motor oil outside the context of an engine. If you put motor oil in your mouth, it would be filthy and slimy and leave a bad taste. But when that oil is inside an engine, it does an important job and you're glad it's there. Weasels are the same way. Slimy and disgusting, but essential."''
-->-- '''[[ComicStrip/{{Dilbert}} Scott Adams]]''', ''The Way of the Weasel''

As a logical extension of TropesAreNotBad, many tropes that might otherwise come across as gratuitous, offensive or just plain wrong in most genres are considered not just accepted in certain genres, but are practically a part of the genre. Complaining about the simple use of the tropes (as opposed to particularly offensive variations) in said genres is rather short-sighted and pointless, since, well, it's in almost ''every'' other work in the genre.

Can often be the cause of an EnforcedTrope. Some AcceptableBreaksFromReality are {{Necessary Weasel}}s to particular genres as well. Contrast with BadWritingIndex, an article listing some real (versus merely perceived) flaws.

Artistic "flaws":
* RuleOfCool in general; "realistic" does not always equal "interesting."
* MoreDakka, StuffBlowingUp in summer action films.
* {{Gorn}} in {{Slasher Movie}}s.
* Blatant WishFulfillment and MarySue characters in {{Fairy Tale}}s and mythology.
* {{Invincible Hero}}es in combat-oriented {{video game}}s. [[NintendoHard Depending on the difficulty at least]] (or the player's skill).
* ToiletHumor in comedies featuring babies.
* An AudienceSurrogate character in a [[HaremGenre Harem]] anime.
* AnticlimaxBoss in WideOpenSandbox games. If a boss must be fought, and players can be optimized for something other than combat, then the boss must be beatable by the weakest character who can reach the fight.
* PurpleIsTheNewBlack, and any other specific attributions of a particular colour to [[LightEmUp light]] or [[CastingAShadow darkness]] (like [[HeavenlyBlue blue]] or [[RedAndBlackAndEvilAllOver red]]).[[note]]Light by default encompasses all non-black colours, so...[[/note]]

* FasterThanLightTravel usually present in space-based ScienceFiction and virtually all SpaceOpera.
** AliensSpeakingEnglish, whenever the plot is not about first contact with them.
** ArtificialGravity, for "[[MohsScaleOfScifiHardness soft]]" ScienceFiction works (though otherwise "hard" film and especially television ScienceFiction will sometimes make use of it due to the extreme difficulty of producing true "zero-g" on camera).[[note]]Although, this may depend on how it's presented and achieved in-story. For example, just spinning the ship and having people walk on the side walls is perfectly reasonable.[[/note]]
** InertialDampening: Often accompanies FasterThanLightTravel (or at least any acceleration/deceleration that can be expressed as ".C"), needed to keep the crew from being rendered a fine paste in high-speed maneuvering or entering/exiting FTL travel.
* NoConservationOfEnergy in portrayals of StockSuperPowers.
* Ignoring of the SquareCubeLaw (as well as other laws of physics) in SuperHero, HumongousMecha, and AttackOfTheFiftyFootWhatever stories, as well as some types of StarfishAliens.
* InstantExpert in {{Role Playing Game}}s.
* BenevolentArchitecture - Sure, there is very slim chance that video game character's surroundings will be perfectly suited to that character's unique powers or abilities. Or that wreckage will form a path, rather than an obstacle. However, being realistic would mean {{Unwinnable}}.
* ThePerryMasonMethod: Because once the audience knows who the real culprit is, there's usually no real reason to continue the story.
* HitboxDissonance in BulletHell games - otherwise they'd be impossible rather than just NintendoHard.
* JumpPhysics in [[PlatformGame platform games]].
* PowerCreepPowerSeep in {{Crossover}} {{Video Game}}s.
* {{Masquerade}} allows the writer to introduce fantastic elements to the story while [[PlausibleDeniability keeping the setting relatable for the audience]].
* Similarly, ReedRichardsIsUseless is necessary to keep the larger setting of a superhero world from transforming into ScienceFiction with leotards.