Psychiatric social work is a complementary activity to the psychiatrist's specialized functions. Both have as their objective the welfare of the mentally ill. It is the result of the evolution of the philosophy and practice of social assistance, either public or private, as well as those new acquisitions obtained in the fundamental concepts of mental diseases. Some modern studies in the correlated fields of Psychology, Sociology, Cultural Anthropology and Social Psychology made the main part in this revision of concepts. The author makes an analysis of the field and function of psychiatric social work, its limits and tecnics, and the organisation and rôles of institutions specialized in this branch of social work. He thinks that more cooperation between psychiatry and social work and a wider spread of social work practices should result in advantage for both. To psychiatry in particular, its share will be a perfectioning of concepts and an increased praticai efficiency.