A Study to Assess Effectiveness and Efficiency of VNS Therapy in Patients With Difficult to Treat Depression.

Inclusion Criteria: * Be at least 18 years of age. * Have a documented primary diagnosis of chronic (\>2 years) or recurrent (2 or more prior episodes) major depressive episode that has not adequately responded to an adequate number of antidepressant treatments, as per local medical standards. This diagnosis must be confirmed using the MINI. * Provide written Ethics Committee (EC) or Institutional Review Board (IRB) approved informed consent and Health Insurance Portability and Accountability Act (HIPAA, US only) authorization (as applicable according to local requirements). * Currently is receiving at least one antidepressant treatment (i.e., antidepressant drug, maintenance electroconvulsive therapy, or formal psychotherapy including supportive psychotherapy) or mood stabilizing treatment for bipolar patients (such as lithium, anticonvulsants, or atypical antipsychotics). * Able and willing to comply with the frequency of (outpatient) clinic visits and to reliably complete all the evaluations as specified in the study protocol.Hence based on the nature of their disease, the following patients should not be included: patients with mental retardation, current severe or significant substance/alcohol abuse, diagnosis of one or more schizophrenia-spectrum or other psychotic disorders, diagnosis of borderline or severe personality disorder as determined by clinical judgment which, in the investigator's opinion, would significantly interfere with subject's participation in the study) Exclusion Criteria: There are no exclusion criteria; the investigator should refer to the (local applicable) VNS Therapy Physician's Manual.