The separation of persistent depression into meaningful and useful subcategories, including major depression, dysthymia, recurrent brief depression, and depressive personality disorder, is the subject of much debate. Depressions can be grouped on the basis of their type and severity of symptoms, aetiology, clinical course, or their association with other psychiatric illnesses. Several investigators have conducted epidemiologic and family studies to evaluate the prevalence of depressive disorders, their diagnostic stability over time, and the amount of overlap among the disorders. Although progress has been made toward a better understanding of the different disorders, insufficient evidence exists to support the hypothesis that these disorders are separate and distinct from one another. However, preliminary data suggest that depressive personality disorder is separate from the other disorders. Additionally, several questions have been raised, particularly the extent to which differentiation between the depressive disorders, specifically major depression and dysthymia, has an impact on treatment decisions.