Psychiatry is a discipline that crosses many frontiers, involving a know­ ledge of the anatomy, physiology and biochemistry of the nervous system, of general medicine, of sociology, of psychology, of the law and of all those subjects which comprise the behavioural sciences. Moreover, in the field of psychiatry the explosion of knowledge has led to an increase in special­ ization in sub-disciplines. To all those interested in psychiatry this diversity causes real problems. They may have to consult a wide variety of sources before finding the answer to what may be in fact a relatively simple question. This encyclopaedia aims at providing straightforward answers to the many questions that may arise. The editors wish to stress that this is not intended to be an advanced treatise on the minutiae of the subject. There will be entries with the contents of which some experts may disagree; others which may seem to err on the side of simplicity or of dog­ matism. For whatever imperfections there may be, the editors take full responsibility. They hope that no major topics have been omitted and that the coverage is sufficiently comprehensive as to provide the reader, whether he be directly involved in psychiatry or not, with a useful and practical guide to the subject.