The author describes his experience as an expert in evaluating defendants convicted of capital murder. He reviews the Supreme Court decisions and provisions of state law that have led defense attorneys to obtain psychiatric evaluations of their clients. Three illustrative cases are presented, one of an incompetent grossly impaired defendant, one of a defendant for whom the finding of mental health played a role in over turning his conviction, and one for a defendant who was later found to be retarded and therefore not executable. The clinical and social implications of psychiatric participation in these cases are discussed.