A history of the application of psychology to legal matters is sketched, and then the legal rules which govern the testimony of a psychiatrist in court are discussed. The following fundamental psychological issues which must be dealt with by the legal system are examined: when the accused is competent to stand trial, what constitutes diminished responsibility, and when the accused is innocent by reason of insanity. The changing legal perception of these issues is explored by examining prominent cases and opinions. The many kinds of mental and emotional aberrations which can lead to criminal behavior are examined, including how the convulsive state can lead to violent crimes, as well as the criminal activity that can result from sexual problems, drug addiction, and family upsets. The impact of psychiatry on civil law is also treated, with the psychological issues that can arise in personal injury, worker's compensation, and other tort actions, as well as in professional liability cases considered in detail. The material is drawn from actual cases. A subject index and a bibliographic essay are provided. (Author abstract modified)