Among the disciplines which are growing at the most accelerated pace is psychiatry, and understandably so. Intimately involved with psychology, general biology, neurology, sociology, cultural changes of any kind, it is constantly enriched by these fields and many others, which all converge toward a greater knowledge of man in health and mental illness. When one looks at the first edition of this Handbook several thoughts emerge: how much of its content has preserved its validity; how little is no longer acceptable; but, most of all, how much is missing from it which we are now in a position to include. And yet the initial two volumes of the first edition appeared in 1959; the third in 1966. The time elapsed is short, but the work done in the field in these intervening years is enormous. A new and larger edition has thus become necessary. The success of the first edition, not only in the United States but throughout the world, gave the inspiration and provided the energy necessary for the preparation of this second edition.