Descriptive studies of schizo-affective disorder can be reviewed to suggest answers to several major questions regarding this disorder. Cross-sectional studies have not supported the existence of a dichotomy between schizophrenia and affective disorders. Rather, they suggest a continuum of disorders with schizo-affective clinical pictures in the middle. Longitudinal studies indicate that some patients with schizophrenia and major affective disorders go through a 'schizo-affective period' on the way to developing the clinical picture of the other disorder, while still other patients have a continuity of syndrome type. These findings suggest the importance of considering disorder processes rather than just disease types. An interactive developmental model of disorder is one way of conceptualizing these processes.