This registry will collect information about patients with treatment-resistant depression (TRD) who are currently in a major depressive episode. For the purposes of this study, TRD is defined as an ongoing depression lasting at least 2 years or that has recurred at least 3 times, to include the current episode, during the patient's lifetime AND has not adequately responded to 4 or more adequate antidepressive treatments. The registry will follow the clinical course and outcomes for patients with TRD who are treated with and without adjunctive (used along with other treatments for depression) vagus nerve stimulation (VNS) therapy.