This review provides an overview of the epidemiology, risk factors, and genetic epidemiology of mood disorders in adults and children. The magnitude and impact of mood disorders in the community outweighs that of most other chronic diseases. Although there is substantial knowledge regarding the sociodemographic risk factors for mood disorders, our understanding of the pathogenesis and classification still is evolving. Comorbidity of mood disorders with anxiety disorders and substance abuse has been documented widely. Whereas substance abuse and mood disorders seem to be independent etiologically, anxiety and mood disorders result from partially common etiologic factors. The results of family, twin, and adoption studies reveal that a positive family history is the most potent risk factor for mood disorders, particularly bipolar disorder. However, the specific factors that are transmitted in families still are unknown. The two areas that will inform future genetic research include phenomenologic studies that refine the validity of the current phenotypic classification of mood disorders, and application of study designs to elucidate specific factors that may explain the familial transmission of these disorders.