Introductory remarks.-Many terms and expressions are in current use in psychiatry which mean different things to different people; and mental hygiene is one of them. It would seem best to refrain from the use of such terms or expressions, formulating what one has to say in simple, unteehnieal, unequivocal language. If, to avoid verbosity or awkwardness, one finds it necessary to use a concise, though more or less vague and ambiguous, expression, it is incumbent on him to define as clearly as possible his meaning of it, in order to avoid misunderstanding. Then there can be no serious objection to his special use of the expression under consideration, for the purpose in hand, even if such use should not represent the unanimous preference of other workers in the field; and under the circumstances, there is certainly no occasion for him to dispute the legitimacy of other workers' preferring to use the same expression in a different sense. It is to be hoped, of course, that, with progress in psychiatry, there will be such a crystallizing out of fundamental concepts that terms and expressions in current use will have only one conventional meaning.In this MANUAL, and particularly in the present chapter, the expression mental hygiene is used to mean the science and practice of the preservation of mental health and efficiency, envisaging a threefold purpose: (a) the securing, as far as may be possible, through eugenics and otherwise, of satisfactory intellectual and temperamental inborn endowments for all persons; (b) such adjustments, physical, educational, vocational, social, sexual, etc., as would result in the fullest and happiest utilization of inborn endowments and capacities; and (c) the prevention of mental disorders.Eugenics.-It is obvious that for mental disorders which are determined solely by hereditary factors the only prevention is through 749