This chapter covers depression. After a historical perspective, it describes the symptoms, caseness, and clinical presentations to aid in diagnosis. Epidemiology and aetiology of the disease are both covered, and specific investigations for diagnosis are outlined. The course and prognosis are discussed, and management principles are covered. Both inpatient and outpatient treatment are covered. Treatment methods for depression both with and without psychotic features are defined, and treatment-resistant depression management is also discussed. Antidepressants, including schedules for swapping or stopping drugs, are covered, including tricyclic antidepressants (TCAs), monoamine oxidase inhibitors (MAOIs), and selective serotonin reuptake inhibitors (SSRIs), as well as assorted other pharmaceutical interventions. Electroconvulsive therapy (ECT) is covered in detail.