Major depression is a widespread, often chronic disorder affecting the individual, his or her family, and society as a whole. It incurs tremendous social and financial costs in the form of impaired relationships, lost productivity, and lost wages. Although chronic major depression is eminently treatable, it continues to be undertreated and underrecognized. This is particularly true in primary care settings, where physicians are usually the first to encounter chronic depression but are seldom trained to distinguish depression from other medical illnesses with similar symptoms. In addition, because of the stigma attached to depression, patients often characterize their symptoms as part of a physical illness or fail to report them to a clinician at all. This article discusses the epidemiology of depression, its impact and burden on society, and its special character (including diagnosis and treatment) as a chronic illness.