Biomarkers of Depression and Treatment Response

Inclusion Criteria: * Age 18-70 * Meet Diagnostic and Statistical Manual-V (DSM-V) diagnostic criteria for Major Depressive Disorder in a current major depressive episode, without psychotic features. * Has Montgomery-Asberg Depressive Rating Scale (MADRS) of \> 19 at baseline, corresponding with moderate to severe depression. * Demonstrate a moderate level of resistance to antidepressant treatment in the current episode, defined as a failure of 1-4 adequate medication trials. * If participant is on a regimen of psychotropic medication, no changes in this regimen should be made during the period between the time at which pre-treatment and post-treatment scans are taken. * Willing and able to undergo non-invasive brain stimulation * Willing and able to attend research visits for approximately 8 weeks * Willing and able to provide informed consent * Ability to speak and read English Exclusion Criteria: * Diagnosed with acute or chronic psychotic symptoms of disorders (e.g. schizophrenia, schizophreniform, schizoaffective disorder) in the current depressive episode. * Has neurological conditions including epilepsy, cerebrovascular disease, dementia, increased intracranial pressure, having a history of repetitive or severe head trauma, or with primary or secondary tumors in the central nervous system. * Presence of an implanted magnetic-sensitive medical device in or near the head, including but not limited to pacemaker, vagus nerve stimulator, or metal aneurysm clips or coils, staples, or stents. * Generalized anxiety disorder as the primary DSM-V disorder during the current MDD episode. * Meets criteria for alcohol or substance abuse or dependence (other than caffeine) in previous 6 months, as determined by the SCID * History of seizures * Implantable hardware not compatible with MRI or with the study * Inability to comply with study daily visits * Women who are pregnant, plan to become pregnant, or breast feeding * Inability to speak and/or read English * Inability to give consent * Any active suicidal intent or plan during the current depressive episode, as determined by a score of 3 on Question #9 of Beck's Depression Inventory (scores reviewed daily by study team members versed in scoring clinical scales), or as by a subjective determination by a study clinician during any study visit.