Chronic affective disorders have generated much interest during the past decade due to increasing recognition of their clinical importance and because of controversy about their appropriate classification and treatment. The purpose of this paper was to review cyclothymic disorder. This review will include historical concepts, clinical phenomenology, family history, biological studies, and treatment of cyclothymia. In addition, the relationship of cyclothymia to other psychiatric disorders, such as bipolar, dysthymic, and personality disorders, is examined. The results suggest that some forms of cyclothymia are strongly associated with bipolar disorder, but that the condition is clinically heterogeneous. The problems of existing research in this area and future directions for research are also discussed.