Over the past 30 years, considerable advances have been made in understanding the origin of depression. The explosion of knowledge includes studies on genetics, animal models of depression, neurotransmitter and neuroendocrine studies, and psychological and sociological risk factors. Depression, like other medical illnesses, presents with a wide spectrum of syndromes and severity. Most clinicians support a risk factor model that reflects the varying contributions of biological (including genetic), psychological, and environmental factors. This article reviews the research on the etiologic factors in depression.