The goals of this study are: 1) to investigate the efficacy of combining ketamine with intensive cognitive behavioral therapy (CBT) to sustain the antidepressant effects of ketamine; and 2) to determine ketamine's delayed effects on learning and memory, and to explore the relationship between any ketamine-induced changes in learning and memory and duration of antidepressant efficacy, with and without CBT augmentation. Subjects with a diagnosis of MDD who are treatment-resistant to at least 2 antidepressants and have chosen to pursue clinical ketamine treatment at Yale Psychiatric Hospital will be recruited for the study.

Inclusion Criteria:

• Suffering from a major depressive episode based on Diagnostic and Statistical manual (DSM) 5 criteria and having failed one or more standard antidepressant treatments during the current episode
• Hamilton Depression Rating Scale (17-HAM-D) score of 21 or more prior to ketamine treatment.
• Planned clinical treatment with ketamine at Yale Psychiatric Hospital (YPH)
• As the purpose of this study is to determine the feasibility and efficacy of CBT to sustain the antidepressant effects of ketamine, only those who achieve a clinical response (i.e., 50% reduction in depression symptoms, as measured by the Montgomery-Asberg Depressive Rating Scale (MADRS) will be eligible for randomization.
• Patients must be treatment resistant to at least two drugs used to treat depression.

Exclusion Criteria:

• Any Axis I or Axis II Disorder, which at screening is clinically predominant to their depressive episode or has been predominant to their depressive episode at any time within 6 months prior to screening
• Active suicidal thoughts with a plan
• Current or recent (<6 months ago) substance use disorder
• Non-affective psychosis (such as schizophrenia or schizoaffective disorder)
• Inability to speak English fluently
• A clinically significant abnormality on the screening physical examination that might affect safety, study participation, or confound interpretation of study results
• Dementia, delirium, or any other neurological or mental disease that might affect cognition or the ability to meaningfully participate in cognitive behavioral therapy (CBT).