Biological evidence in forensic science includes organic materials, such as blood, semen, hair, saliva and skin tissue. Forensic scientists use fingernail scrapings and bone to identify victims and criminals. Biological evidence is extremely important in cases of sexual assault and violent crimes.
Forensic scientists are able to find and recover extremely small biological samples from crime scenes. This is based on "Loclard's Exchange Principle." The basis of the principle is that every time a person enters a room or environment, they take something with them and leave something behind, such as biological evidence. Even simple contact between two people can leave biological evidence.