A given set of circumstances which comprise a crime scene is never left wholly unchanged by the commission of a crime and the passage of the perpetrator to or from that scene. The value of the changes caused in the scene by a suspect can be in the demonstration that a crime has been committed, in the identification of the person or persons who did it, and in exonerating all other persons who may be under suspicion. The analysis of physical evidence involves first placing an object or finding in a class or group and then determining some characteristic that makes it unique. The collection of physical evidence from a crime scene requires that the first officer on the scene be responsible for securing the crime scene from unauthorized intrusion. Without this protection, all evidence within the scene is subject to question. Specifically, precautions must be taken to preserve areas likely to bear foot, shoe, tire, or finger impressions, and efforts should be made to avoid disturbing bodies or the removal of any evidence without authorization of the officer in charge of the scene. The investigation of an actual homicide case in California illustrates the procedures for collecting evidence and the logic used to make a successful case from evidence. Seven footnotes are listed.