This is a 60-day, multi-center, double-blind, placebo-controlled study on the efficacy of aripiprazole (Abilify) augmentation of selective serotonin reuptake inhibitors (SSRIs) or selective serotonin norepinephrine uptake inhibitors (SNRIs) venlafaxine in patients with MDD who have responded inadequately to treatment with ADTs. The purpose of our study is to evaluate the efficacy and tolerability of low-dose (2 mg/day) aripiprazole (Abilify) as an augmentation strategy in MDD non-responding to ADT with SSRIs or the SNRI venlafaxine.