• The primary efficacy endpoint is the QIDS-C change. [ Time Frame: From Baseline to Study Week 22 ] [ Designated as safety issue: No ]
  

• The key secondary efficacy endpoints are the change for the IDS-C, MADRS and IDS-SR [ Time Frame: From Baseline to Study Week 22 ] [ Designated as safety issue: No ]
  
• Safety analysis will be performed on all adverse events. [ Time Frame: Entire course of study ] [ Designated as safety issue: Yes ]
  

This is a postmarket medical device study. This study will examine treatment outcomes for patients with TRD who are randomized to VNS Therapy administered at different amounts of electrical charge. The Sponsor, Cyberonics, provides funding for this study. Patients are followed for 54 weeks, 50 of those weeks are following implantation of the VNS Therapy system. No study sites will be permitted to enroll study subjects until Institutional Review Board (IRB) approval has been received. Sites are permitted to be approved by a local or a central IRB.

Inclusion Criteria:

• Patient has a diagnosis of chronic or recurrent depression and is currently experiencing a major depressive episode.
• Patient has not had an adequate response to four or more adequate antidepressant treatments from at least two different antidepressant treatment categories.
• Patient has (in the investigator's judgment) sufficient impairment from his/her depression and/or depression treatment that the potential benefits/risks of VNS Therapy are warranted.
• Patient must currently be receiving at least one antidepressant treatment; the patient must be receiving all current antidepressant treatments in a stable regimen.
• If the patient has a current diagnosis of bipolar disorder, the patient must be receiving a mood stabilizer.
• Patient must be 18 years of age or older and of legal age of consent.
• Patient must be able to complete the evaluations specified in the study procedures flow chart.
• Patient must provide written informed consent and Health Insurance Portability and Accountability Act (HIPAA) authorization.

Exclusion Criteria:

• Patient has had a bilateral or left cervical vagotomy.
• Patient currently uses, or is expected to use during the study, short-wave diathermy, microwave diathermy, or therapeutic ultrasound diathermy.
• A VNS Therapy System implant would pose an unacceptable surgical or medical risk for the patient.
• Patient is expected to require full body magnetic resonance imaging during the clinical study.
• Patient is acutely suicidal.
• Patient has a history of schizophrenia, schizoaffective disorder, or other psychotic disorder or a current major depressive episode that includes psychotic features (commonly referred to as psychotic depression).
• Patient has a history of rapid cycling bipolar disorder or a current diagnosis of bipolar disorder mixed phase.
• Patient has a history of borderline personality disorder.
• Patient has a history of drug or alcohol dependence within the 12 months prior to the baseline visit or currently takes a narcotic drug five or more days per week.
• Patient is currently enrolled in another investigational study.
• Patient has had a prior VNS Therapy System implant.

Note: Some IRBs may require additional conditions for enrollment.