Psychiatric evidence is admissible at two stages of the trial: determination of guilt and sentencing. This evidence is usually relevant in one of five areas: fitness to plead, the insanity defense, diminished responsibility in homicide cases, automatism, and drunkenness. Fitness to plead rarely arises in practice, but when it does the matter is governed by the common law test, namely, the ability of the accused to understand the charge to distinguish between the plea of guilty and not guilty, to exercise the right to challenge jurors, and generally to understand the proceedings. Only a jury, especially empaneled to try that specific issue, can determine fitness to plead. There is a right of appeal from such decisions. In case of the insanity defense, the M'Naghten Rules, limiting the evidence to the issue of cognition, is applied. However, the insanity defense is rarely used in English trials today. Diminished responsibility can be used to reduce murder to manslaughter, and applies a broader rule than M'Naghten -- mental abnormality which influenced behavior. In rebuttal, the prosecution can claim that it is really a case of insanity and that M'Naghten should apply; the defense case will be considerably bolstered by evidence of past or present psychiatric treatment for a disturbance. Automatism is a relatively recent addition to the psychiatric defenses to crimes, and is available in two forms -- insane in the cases where mental abnormality can be shown, and sane in the cases where the defendant is under the influence of sudden and overwhelming impulse ceated by a trauma like a blow to the head. Voluntary drunkenness does not aid a criminal defendant attempting to establish a diminished capacity defense. The largest use of psychiatric reports or testimony is in mitigation hearings for sentence reduction. The psychiatric report could lead to mandatory treatment of the defendant, psychiatric probation, discretionary reduction of sentences from mandatory life imprisonment for certain crimes to a term of years, and reduction of sentence on appeal. Footnotes are supplied.