This paper reviews the current position of studies on the epidemiology of bipolar affective disorder. A disorder that cannot be recognized until sometime after its onset poses special difficulties for epidemiological study. These are discussed and attempts made to solve them. Community psychiatric surveys suggest a morbid risk of bipolar disorder of around 2-2.5%, but probably include many false-positives. Studies of treated cases indicate a morbid risk of 0.5%, but will miss untreated cases. It is probably reasonable to suggest a compromise value of 1-1.5%; bipolar disorder is thus still a rare condition. It is possible to quantify the unipolar-bipolar conversion rate, which is of the order of 5%, and is of particular interest that female sufferers have proportionately fewer manic episodes. Age at onset, possible cohort phenomena, comorbidity, and sociodemographic correlates are discussed.