There is a growing body of evidence that supports the use of atypical antipsychotics as augmentation agents for nonpsychotic unipolar major depressive disorder (MDD) in adults. Unfortunately, varying definitions of treatment-resistant depression, the limited evidence available for interventions after two or more treatment failures, and when and whether to use medications from nonantidepressant classes, remain a key gap in the knowledge base for clinicians. We identified and reviewed the following guidelines to discuss the status of augmentation therapy with atypical antipsychotic agents in MDD: American Psychiatric Association practice guidelines for treatment of patients with MDD; Canadian