Major depressive disorder (MDD) is a debilitating illness affecting 7% to 12% of men and 20% to 25% of women. It is usually a recurrent illness, with up to 30% of patients experiencing a depressive episode lasting over 2 years. Depression may also increase the morbidity and mortality of numerous medical conditions, such as cardiac disease, myocardial infarction, chronic pain, diabetes, cerebrovascular events, and respiratory illnesses. The goal of antidepressant therapy is to achieve full remission and functional recovery, and continuing treatment beyond the acute phase is usually necessary to maintain remission. In contrast with full remission, individuals who experience residual symptoms, however mild, have a higher chance of experiencing one or more additional episodes.