This will be a multicenter, randomized, double-blind study designed to assess the efficacy, safety and tolerability of an oral Aripiprazole/Escitalopram combination therapy in subjects with MDD who have demonstrated an incomplete response to a prospective trial of Escitalopram, and report a treatment history for the current MDD episode of an inadequate response to at least one and no more than three adequate trials of an approved antidepressant other than Escitalopram. An inadequate response is defined as less than a 50% reduction in depressive symptom severity as assessed by the subject's self-report on the Massachusetts General Hospital Antidepressant Treatment Response Questionnaire (ATRQ) and evaluated by the investigator as part of the subject's medical and psychiatric history. An adequate trial is defined as an antidepressant treatment for at least 6 weeks duration (or at least 3 weeks for combination treatments) at an approved dose as specified in the ATRQ.