One of the most robust findings in psychiatric epidemiology is that females are twice as likely to experience depression—a disorder of mood characterized by sadness, anhedonia, or, in children and adolescents, irritability—as males, a disparity that emerges during early adolescence. This entry reviews differences between girls and boys in the prevalence and presentation of depression, identifies mechanisms explaining these differences, and describes the impact of gender and sex (i.e., gender being the characteristics of biological status imposed by a given society and sex being one’s biological status) on prognosis and treatment.