Recent reports of a rapid antidepressant effect of the glutamate N-methyl-D-aspartate (NMDA) receptor antagonist ketamine, even in treatment-resistant populations, have spurred translational therapeutic and neuroscience research aimed at elucidating ketamine's mechanism of action. This article provides a concise overview of research findings that pertain to the effects of low-dose ketamine at the cellular, neurocircuitry, and behavioral levels and describes an integrated model of the action of ketamine in the treatment of depression.