Inclusion Criteria:

1. Outpatients or inpatients with nonpsychotic MDD.
2. A score of >14 on the HRS-D17 (equivalent to 10 or greater on PHQ-9 which is used in primary care to assess depression) given that when medication exceeds the effect of placebo in primary care participants have a HRS-D17 >12. We added 2 HRS-D17 points to take into account the possibility of measurement error.
3. Outpatients or inpatients for whom antidepressant treatment is deemed appropriate by the treating clinician.
4. Subjects who are between 18-85 years of age.
5. Participants who have general medical conditions (GMCs) which could conceivably be physiologically causing their depressive symptoms will receive treatment as usual for their GMCs as well as for their MDD.

Exclusion Criteria:

• Subjects with medical contraindications that preclude citalopram or escitalopram treatment and those who have previously failed to respond to citalopram or escitalopram will be excluded. In addition, patients with schizophrenia, schizoaffective disorder, or who have Bipolar I disorder will be excluded because they have a primary psychiatric condition that requires a different initial treatment. Subjects currently on antidepressant medication with subtherapeutic results in terms of depression management will undergo a medication taper and discontinuation prior to initiation of citalopram or escitalopram treatment. The subject will be closely monitored by the primary physician or psychiatrist during the medication taper and discontinuation phase. The medication taper is left upto the treating physician's or psychiatrist's discretion. Study subjects who cannot be safely tapered from their medication or experience adverse effects during the taper will be excluded from the study. Study subjects using their antidepressant medication for management of nicotine dependence, chronic pain, migraine prophylaxis or other diagnoses will not be eligible for the study. Trazodone, Melatonin, and Diphenhydramine may be used as rescue medications for insomnia. Benzodiazepines may be used for treatment of anxiety and atomoxetine may be used for the treatment of attention deficit disorder. Study subjects currently on antipsychotic medications (e.g., typical and atypical antipsychotic drugs) and mood stabilizing agents (e.g., lithium, carbamazepine, valproate, lamotrigine, gabapentin, or other anticonvulsants) are not eligible for the study with the exception of those starting quetiapine after baseline. Subjects unable to give informed consent are excluded. Pregnant subjects will be excluded.