This is a study of the effectiveness of adding Abilify (aripiprazole), an atypical antipsychotic medication, to ongoing selective serotonin reuptake inhibitor (SSRI) antidepressant treatment for depressed outpatients who are not responding fully to SSRI treatment alone. It is hypothesized that patients' functioning will improve after 12 weeks of treatment with Aripiprazole and SSRI medication.