Understanding the epidemiology of major depressive disorder (MDD) and the neurobiologic theories behind depression and antidepressant treatment is vital for physicians who must identify and treat patients with this disorder. The epidemiology of MDD reveals that this disorder is widespread: the lifetime prevalence of MDD is estimated to be ∼17% and the 12-month prevalence is ≥7%, according to the National Comorbidity Survey Replication. Epidemiologic studies suggest that in any 30-day period, 2% to 5% of the United States population meet criteria for MDD. In addition, nearly twice as many women as men (21% versus 13%, respectively) are affected by a depressive disorder during their lifetimes. These numbers reveal a vast population of people affected by MDD, making depression a tremendous social and medical concern.