Physical evidence is any tangible article, small or large, that relates to a determination of what happened at the crime scene. Physical evidence can contribute to the reconstruction of the crime, the identification of the participants, and the confirmation or discrediting of a suspect's alibi. The proper collection and disposition of physical evidence at the crime scene and from the body of the victims is critically important to the investigation and the presentation of the case in court. Classifications of physical evidence are class evidence, which cannot be forensically identified with a specific source to the exclusion of all others (e.g., non-DNA analysis of blood, soil, semen, glass, saliva, and wood); individualistic evidence, which can be positively and forensically identified with a specific source to the exclusion of all other sources (e.g., DNA analysis of body fluids and hair, latent prints, fracture matches, and bite marks); and pattern evidence, which is produced by contact (e.g., blood splatter, glass fracture patterns, and fire burn patterns). Transfer evidence is generally produced by physical contact of persons, objects, or between persons or objects. To be introduced as physical evidence in a trial, an article must be properly identified, show a proper chain of custody, be material and relevant, and meet all legal requirements. Each piece of evidence should be marked to show its original position and location and also recorded in the investigator's notebook. The searching officer should mark each article to identify the person who found the particular piece of evidence. Each piece of evidence should show proper disposition regarding custody and analysis. Each item should be photographed before it is collected and removed from the crime scene. An excellent procedure for the photographic visualization of sexual assault injuries is through the application of Toluidine Blue in .01-percent solution, or Methylene Blue or Azure. A color Polaroid photo taken before and after the application of these substances produces excellent documentation of sexual abuse. Coupled with proper evidence collection and custody procedures, an investigator's knowledge of human behavior patterns and psychosexual activities provides additional insight in determining the reason and motive for a sex-related homicide.