The role of the trial participants -prosecutors, magistrates or judges, grand juries, trial judges, juries, and appellate courts -- is reviewed. Ways that officers can enhance courts -- is reviewed. Ways that officers can enhance their credibility, including listening carefully to each question, are delineated. Also noted are ways to avoid suppression hearings -- tactical devices in which defense counsels attempt to keep vital evidence from the jury by having the judge declare that it was illegally or unconstitutionally provided. For instance, officers should complete search warrants as completely and specifically as possible. Officers should also discard all case materials except facts that tend to prove the defendant guilty or innocent or that tend to ameliorate or enhance punishment. Additionally, they should be aware of the basic rules of evidence, including res gestae, declarations against penal interest, dying declarations, business records, and laboratory reports. Familiarity with the basic patterns which prosecutors use to discredit testimony, including limiting officers' ability to know the truth, is essential. Knowledge of special problems that may be faced in testifying and in cross-examination is recommended. These problems concern inconsistent statements, police use of force, sex crimes, informants, political cases, questionable confessions, motion in limine, out-of-court statements, personal contact, professional rivalry, disappearing victims, and missing witnesses. Finally, officers should understand the most important concepts in criminal justice: presumptions, inferences, and circumstantial evidence. An index is provided. Jury instructions for murder in the first degree and a case history are appended.