From: http://www.profiling.org/journal/vol1_no1/jbp_ed_january2000_1-1.html "Forensic scientists have almost universally accepted the Locard Exchange Principle. This doctrine was enunciated early in the 20th Century by Edmund Locard, the director of the first crime laboratory, in Lyon, France. Locard's Exchange Principle states that with contact between two items, there will be an exchange." Essentially his principle is applied to crime scenes in which the perpetrator (s) of a crime comes into conact with the scene, so he/she will both bring something into the scene and leave with something from the scene. Every contact leaves a trace.

Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value. - Professor Edmond Locard

‘Every contact leaves a trace’ is known as Locard’s Principle. Dr Locard worked in the Technical Police Laboratory in the 1900s and said, “There is no such thing as a clean contact between two objects. When two bodies or objects come into contact, they mutually contaminate each other with minute fragments of material…” He said that the microscopic debris on our bodies and clothing was a “witness sure and faithful of all our movements and encounters”. The fundamental principle of forensic science that "Every Contact Leaves a Trace," is a simple truth which underscores all processes of forensic examination, beginning at the scene of a crime and continuing through to the recovery of trace substances from items of physical evidence in the forensic laboratory. At every crime scene, the criminal unwittingly leaves clues behind, and also takes things away. This evidence is often microscopic, such as clothing fibres, plant pollen, soil particles, or tiny fragments of glass or paint. The job of forensic scientists is to compare samples from the scene with those taken from a suspect. Each match makes it more likely that the suspect was at the scene.

it is the idea that suspects/perpetrators bring items of evidence into the crime scene and will take itms with them when they leave. There is an exchange of trace evidence that includes hair, fibers, dirt, dust, blood, body fluids, skin cells and other microscopic materials