''Every contact leaves a trace. Every time one object comes in contact with another object, it takes something from that object or leaves something behind.''

Principle which underwrites modern forensic science. The reason that criminalists look for fingerprints, DNA, soil samples, fibers, and all the evidence that falls into the category of "trace".

In real forensics, this is a very general principle from which we learn nothing more than that it makes sense to look for trace evidence.

In TV, it is ''always literally true''. Every crime scene has an abundance of fibers, soil, DNA, bodily fluids, and [[FingerprintingAir fingerprintable air]].

This representation of the Locard principle is one of the reasons for TheCSIEffect.

In works that portray FunctionalMagic, Locard's Theory is a subset of the magical principle known as the [[http://www.themystica.com/mystica/articles/l/law_of_contagion.html Law of Contagion]].

* That a man cannot stay somewhere without leaving some kind of trace is a mantra of Inspector Lunge in ''Anime/{{Monster}}''. Of course when it comes to Johan, it's not that simple.
** In fact, this is part of what gets him to trust the doctor's story about Johan. He says something earlier about how "nothing short of a demon" could go out of an area without leaving a trace. Johan apparently was in the area, but his room is so totally clean that he is in fact the "demon" of the story.
** Naoki Urasawa uses this trope in another of his suspense manga, the ''Manga/AstroBoy''-inspired ''Manga/{{Pluto}}''. The fact that no trace evidence can be found at any of the murder scenes leads the investigators to conclude that the SerialKiller they're looking for is a robot.
* Put to good use in ''Manga/DeathNote''; the reason it's so difficult to track down Kira is that the [[ArtifactOfDoom Death Note]] kills without leaving any physical traces, so L had to look for a culprit based on why the killings occurred the way they did. On the other hand, L was able to use forensic evidence against Misa because of the traces she left on the Second Kira tape, even though she had arranged for someone else to get their fingerprints on it.

* Central to ''Film/{{Gattaca}}''.

* Mentioned numerous times in Jeffrey Deaver's Literature/LincolnRhyme novels.
* Robert Fulghum mentions this by name in one of his ''All I Really Need To Know I Learned In Kindergarden'' books, then expands it in a philosophical direction. Thus, everyone influences numerous things in a small way as they pass through life.
* ''The Fruit at the Bottom of the Bowl'' has the main character take the idea of this to extremes-after he kills a man, he cleans the entire house meticulously from top to, well, the fruit at the bottom of the bowl.
* Used in a form in the Literature/{{Discworld}} novel ''Discworld/FeetOfClay''. Golems don't have fingerprints, but they do have traces of where they usually hang out.
* In several of HarryTurtledove's works, magic runs on two key laws: the Law of Contagion (this) and the Law of Similarity.

* Possibly the earliest TV instance of this is an episode of ''Franchise/{{Dragnet}}'' where Sgt. Joe Friday was accused of shooting a suspect unprovoked. His dogged insistence on "he must have left ''some''thing behind" caused investigators to eventually discover the bullet the suspect had fired. They realized that a line under a shelf was ''not'' a carpenter's pencil mark, but a trace from the bullet. It had pushed the shelf up just enough to hide the bullet hole
** [[FridgeLogic The question]] of why the suspect's gun would appear unfired is left as an exercise to the viewer. Regardless of whether it was a revolver or automatic an unfired round would still be present, in the form of a spent shell casing lying somewhere or inside the revolver. In any case, drawing a weapon against a police officer is a [[SuicideByCop death sentence most of the time]], whether the cop fired first is irrelevant.
* Mentioned by name at least twice in ''CSIMiami''.
** And once by the ''villain'' in ''{{CSI}}''.
*** And at least once by Grissom.
* In one episode of ''Series/StarTrekVoyager'' there was NO TRACE at all on the crime scene. Not a single molecule out of place, handily detected by their EverythingSensor. The murderer was [[spoiler: a hologram.]]

[[folder: RealLife]]
* Subversion: [[http://en.wikipedia.org/wiki/Danielle_Van_Dam The 2002 murder of Danielle van Dam.]] David Westerfield, the neighbor who was indicted for her murder, had a lawyer who used Locard's Theory as his entire case. It didn't work, and now Westerfield's in San Quentin. (Granted, the guy probably wasn't trying very hard to build a case, given Westerfield's attitude...)

[[folder: VideoGames]]
* In ''VideoGame/NeverwinterNights2'', your investigation into the Ember massacre partly involves a magical version of this.