Hair evidence is present in many criminal cases which come to the attention of the Federal Bureau of Investigation Laboratory and can be used to connect a weapon to a violent crime as well as to identify the offender or victim. Hairs are very resistant to decomposition and putrefaction, and no search for this type of evidence can be too meticulous. Following a description of the physical construction of hair, characteristics are detailed that allow the laboratory analyst to determine the human or animal and racial origins of a hair. Differences that hairs from various parts of the body exhibit are also discussed. Examination can show if a hair was pulled out or fell, was crushed, and if it was dyed or bleached. It is not possible by microscopic examination to determine definitely if a particular hair sample came from a particular person. It can be shown, however, that hair of an unknown source matches a known hair sample from a certain individual in all microscopic characteristics and thus could have come from that source. Items that should be investigated in a microscopic examination are listed. The examiner should have a reference file of sample human and animal hairs and have representative samples of hair from the victim and suspect which include at least a dozen hairs from various body areas. Techniques for removing hairs from the crime scene and protecting samples are described. Several photographs accompany the article.