Inclusion Criteria:

1. Age > 60 years.
2. Major depressive disorder (MDD), single or recurrent, as diagnosed by the SCID-IV.
3. MADRS ≥ 15.

Exclusion Criteria:

1. Inability to provide informed consent.
2. Depressive symptoms not severe enough (i.e., MADRS < 15) at the baseline assessments.
3. Dementia based upon DSM-IV criteria as well as a Folstein MMSE score of less than 24. Patients screened out due to dementia will be referred to a memory clinic or to the UPMC Alzheimer's Disease Research Center for evaluation to clarify the presence or absence of a dementia.
4. Lifetime diagnosis of bipolar I or II disorder, schizophrenia, schizoaffective disorder, schizophreniform disorder, delusional disorder, or current psychotic symptoms, as diagnosed by the SCID. A recommendation for psychiatric referral will be made in these cases.
5. Abuse of or dependence on alcohol or other substances within the past 3 months as determined by SCID, and confirmed by study physician interview.
6. High risk for suicide (e.g., active SI and/or current/recent intent or plan) AND unable to be managed safely in the clinical trial (e.g., unwilling to be hospitalized). Urgent psychiatric referral will be made in these cases.
7. Contraindication to venlafaxine XR or aripiprazole as determined by study physician including history of intolerance of either venlafaxine XR or aripiprazole in the study target dosage range (venlafaxine XR at up to 225 mg/day; aripiprazole at up to 15mg/day).
8. Failure to respond to at least 6 weeks of venlafaxine (>225 mg/d) plus aripiprazole (>10 mg/d).
9. Inability to communicate in English (i.e., interview cannot be conducted without an interpreter; subject largely unable to understand questions and cannot respond in English).
10. Non-correctable clinically significant sensory impairment (i.e., cannot hear well enough to cooperate with interview)
11. Unstable medical illness, including delirium, uncontrolled diabetes mellitus, hypertension, hyperlipidemia, or cerebrovascular or cardiovascular risk factors that are not under medical management. This will be determined based on information from the patient's personal physician's and study physician clinical judgment. Referral to the patient's personal physician or to a general practitioner will be made in these cases.
12. Subjects taking psychotropic medications that cannot be safely tapered or discontinued prior to study initiation: this would include patients on Monoamine Oxidase Inhibitors (MAOI) who would need to be off the MAOI for 14 days to be eligible for the study to avoid adverse drug interactions.

Patients will not be allowed to take antidepressant or atypical antipsychotic medication other than the study medication, unless it is a low dose antidepressant prescribed for chronic pain that would not be medically advisable to stop (e.g., amitryptyline 50mg). If a patient's depression is adequately treated on his/her psychotropic medication, he/she would not be eligible for the study. If a patient failed a trial of venlafaxine (12 weeks of treatment with venlafaxine including at least 6 weeks on 300mg/day), he/she would not be eligible. The following are allowed: benzodiazepines up to 2mg/d lorazepam equivalent; other sedative-hypnotics (e.g., zolpidem, zaleplon, eszopiclone); gabapentin if prescribed for non-psychiatric indication (e.g., neuropathy). Except for MAOIs, there is really no clinical rationale to exclude patients on specific concomitant medications unless they are medically unstable (in which case they are excluded from participation). As noted, patients on an MAOI would need to be off the MAOI for 14 days to protect from adverse drug interactions.