DSM-IV major depressive disorder (MDD) is a clinical syndrome notable for heterogeneity of its clinical presentation, genetics, neurobiology, clinical course, and treatment responsiveness. In an attempt to make sense of this heterogeneity, clinicians and researchers have proposed a number of MDD "subtypes" based on differences in characteristic symptoms (e.g., atypical, melancholic, psychotic), onset (e.g., early vs. late, post-partum, seasonal), course of illness (e.g., single vs. recurrent, chronic, double), and severity. This article provides a brief review of the status of several of the most common subtypes in terms of their clinical features, biological correlates, course of illness, and treatment implications.