Depression, a common and disabling condition, is often misunderstood by patients, family members, and clinicians. It is frequently underdiagnosed and untreated or inadequately treated. Criteria for major depressive disorder are listed in the DSM-IV-TR, but even less severe depression may merit intervention--especially if chronic. Our understanding of the etiology of depression is rudimentary, but it may involve multiple genes combined with negative life experiences. A variety of pharmacologic and psychosocial treatments are available for treating depression. Most patients who are well treated can be relieved of symptoms and return to full function.