Considerable literature supports the hypothesis of dysfunction in central nervous system serotonergic circuits in the pathophysiology of mood disorders, specifically major depression. Since the development of the selective serotonin (5-HT) reuptake inhibitors, a putative role for the 5-HT transporter (SERT) in the etiology of depression has been explored. The discovery of a functional SERT polymorphism has provided a novel tool to further scrutinize the role of serotonergic neurons in depression. This article reviews the burgeoning evidence of an interaction between early life stress and an SERT polymorphism on vulnerability to depression.