The psychiatric literature contains numerous accounts of psychoses found worldwide that do not fit classic definitions of schizophrenia, manic-depression, or organic brain syndrome. These vaguely defined 'atypical psychoses' are under scrutiny because of growing knowledge and more rigorous methodology in descriptive and cross-cultural psychiatry. The authors trace the history of atypical psychosis concepts from the contributions of Kraepelin, Jaspers, and Freud to contemporary European and Anglo-American viewpoints. Studies of the atypical psychoses are reviewed in the light of current validation methods in psychiatry. Several methodologic problems plague this area: inadequate definitions, etiologic prejudice, poor premorbid and follow-up assessment, as well as certain naive notions about psychopathology in different cultures. Consequently, knowledge of atypical psychosis is limited and largely impractical. The authors suggest several possibilities for interpreting present and future evidence about these disorders and certain strategies for future studies.