An introduction explains the relation between psychology and the law and presents a definition of forensic psychology especially as practiced in the United Kingdom. The clinical, experimental, actuarial, and advisory roles in forensic psychology are delineated through an examination of case studies, and the various courts of law and their historical development are discussed, with particular reference to the forensic psychologist's involvement with each type of court. The law of evidence relevant to the expert witness in psychology is summarized, with attention to evidence and proof, presumptions, and the hearsay rule. A discussion of the problems of testifying in court focuses on attendance, subpoenas, professional confidence, and examination under oath. Practitioners of forensic psychology in the United Kingdom are admitted to the court to provide expert testimony from the standing of their own professional specialty, whether clinical, experimental, educational, social, criminological, or occupational; they are not there as experts on evidence in general. Nevertheless, knowledge of legal concepts and of the rules of evidence is advised to enable the psychologist to make an optimally effective contribution to the court proceedings. Finally, the principles underlying fees for forensic services are discussed. Tabluar data, charts, a list of related journals, 235 references, a name and subject index, and an index of cases and statutes are included. (Author abstract modified)