This is an eighteen week study including a two week taper off period. Forty five subjects will be started on 10mg escitalopram then titered to 20mg plus placebo. After 10 weeks of treatment, those subjects who do not respond to the escitalopram, as defined by a 50% reduction in their MADRS score, will be started on adjunctive aripiprazole at 2mg, titered to 10mg. Subjects will remain on the both the escitalopram and aripiprazole for 6 weeks. At week 10 prior to starting the adjunctive aripiprazole and week 16 (end of treatment) the subjects will receive the PET and MRI scans. The neuroimaging will consist of fMRI, a raclopride PET scan, and a fluoro-dopa PET scan.