The diagnosis of cycloid psychosis has a long tradition in European psychiatry. However, it has been poorly assimilated within the DSM IV and ICD-10 diagnostic systems. Leonhard set the basis for the current conceptualization of the disorder, and Perris and Brockington developed the first operational diagnostic criteria. However, the two conceptualizations of the disorder are not the same and differ across a number of meaningful variables. Cycloid psychosis is a useful concept in that it possesses both clinical and predictive validity. Despite the high prevalence of mood symptoms and syndromes, cycloid psychosis does not equal schizoaffective disorder. Although a substantial body of evidence suggests that cycloid psychosis differs meaningfully from typical schizophrenia, it is less clear whether it differs from major mood disorders or represents an independent nosological entity. The existence of putative subtypes is also likely, and the differentiation between affective and nonaffective subtypes has received some support.