Integration of addiction into the theory and practice of psychiatry has been hampered by the lack of a definition of addiction which is scientifically useful. A definition is proposed, with diagnostic criteria specified in a format similar to that of DSM-III-R. Essentially, addiction designates a process whereby a behavior, that can function both to produce pleasure and to provide escape from internal discomfort, is employed in a pattern characterized by (1) recurrent failure to control the behaviour (powerlessness) and (2) continuation of the behaviour despite significant negative consequences (unmanageability). Some practical and theoretical implications of this definition are then explored.