Major depressive disorder (MDD) is often characterized by anhedonia and impaired ability to modulate behavior as a function of rewards. However, the neurobiology of anhedonia and reduced reward responsiveness remains largely unknown. Because dopamine (DA) plays a critical role in goal-directed behavior and reinforcement learning, DA dysregulation might play an important role. In fact, several lines of evidence suggest that down-regulation of DA transmission might characterize depression vulnerability and the emergence of depressive symptoms. The current study seeks to elucidate the role of DA dysfunction in MDD. We hypothesize that MDD subjects will show reduced DAT binding potential, reduced reward learning in the probabilistic reward task, and abnormal functional magnetic resonance imaging (fMRI) activation in dorsal and ventral striatal regions during an instrumental learning task.