The roles of the forensic psychologist as expert witness are as experimental psychologist, actuary, and medical witness. In the first of these roles, the experimentalist can offer evidence in three ways. He/she can comment on experimental data from the general body of psychological knowledge, provide evidence from his/her own laboratory studies, or report on field studies he/she has conducted in relation to the case at issue. In the role of actuary, the psychologist can provide the court with data which gives the probability of a particular event or circumstance happening or the mean value of something. The psychologist as actuary is basing calculations on all the relevant observations made by psychologists through the years, but in particular cases, he/she may go into the field for observations. Probability statments, however, are used less in court than in helping the police to decide whether to pursue a particular line of investigation. In the role of medical witness, the psychologist may provide supplementary or complementary evidence bearing upon other medical evidence (for example, to the neurologist in compensation cases, or to the psychiatrist in criminal cases), or provide independent evidence. In civil cases, the forensic psychologist is most often concerned with compensation cases. The psychologist also may function in the court of chancery when an appellant is trying to gain freedom from the court of protection. The psychologist may be asked to testify about the appellant's ability to handle his/her own affairs. Other legal contexts where the psychologist may contribute medical evidence are mental health tribunals, divorce proceedings, and criminal trials, where the psychologist may be concerned with the application of M'Naghten rules and the legal concept of diminished responsibility. Notes and seven references are provided. For related documents, see NCJ 70739 and 70741-48.