Locard's Theory

"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 fingerprintable air.

This representation of the Locard principle is one of the reasons for The CSI Effect.

In works that portray Functional Magic, Locard's Theory is a subset of the magical principle known as the Law of Contagion.

Anime

 * That a man cannot stay somewhere without leaving some kind of trace is a mantra of Inspector Lunge in 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 Astro Boy-inspired Pluto. The fact that no trace evidence can be found at any of the murder scenes leads the investigators to conclude that the Serial Killer they're looking for is a robot.
 * Put to good use in Death Note; the reason it's so difficult to track down Kira is that the 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.

Film

 * Central to Gattaca.

Literature

 * Mentioned numerous times in Jeffrey Deaver's Lincoln Rhyme 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, we all influence numerous things in a small way as we 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 Discworld novel Feet of Clay. Golems don't have fingerprints, but they do have traces of where they usually hang out.

Live Action TV

 * Possibly the earliest TV instance of this is an episode of Dragnet where Sgt. Joe Friday was accused of shooting a suspect unprovoked. His dogged insistence on "he must have left something 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
 * Mentioned by name at least twice in CSI: Miami.
 * And once by the villain in CSI.
 * And at least once by Grissom.
 * In one episode of Star Trek Voyager there was NO TRACE at all on the crime scene. Not a single molecule out of place (I don't know how they could have checked that, especially because it happened in the woods). The murderer was
 * With their Everything Sensor, I presume.

Real Life

 * Subversion: 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...)