The beneficial and harmful effects of acting out are studied in a series of short essays by numerous authors. Included are four articles on the theoretical and dynamic considerations of acting out, along with five clinical manifestations of acting out involving suicide and criminality in adolescents and adults. Special forms of harmful acting out including drug addiction, alcoholism, psychosomatic illness, obesity, learning inhibition, and homosexuality are presented. Group therapy and the management of acting out problems in adolescents and adults are discussed as aids to mental hygiene. The beneficial aspects of predicting acting out by tests, such as the Wechsler Scales, the Rorschach, the Thematic Apperception Test, projective drawings, and the Bender Gestalt are included. (JM)