Electroconvulsive therapy (ECT) involves the application of a brief electrical stimulus to the scalp to induce a generalized tonic-clonic seizure lasting 25-60 seconds under general anesthesia with neuromuscular blockade. The therapeutic mechanism is not fully understood but involves multiple neurobiological effects: enhanced serotonergic, noradrenergic, and dopaminergic neurotransmission; increased BDNF (brain-derived neurotrophic factor) promoting neuroplasticity and neurogenesis in the hippocampus; normalization of hypothalamic-pituitary-adrenal (HPA) axis hyperactivity; anticonvulsant effects through progressive seizure threshold elevation; and enhanced GABAergic inhibition. ECT is the most effective treatment for severe, treatment-resistant major depressive disorder (response rates 70-90% versus 20-30% for medication in treatment-resistant cases), catatonia (response rates >90%), and acute suicidality when rapid response is needed. It is also indicated for psychotic depression, mania refractory to pharmacotherapy, and neuroleptic malignant syndrome. The clinician evaluates candidacy (including cardiac and anesthetic risk), selects electrode placement (right unilateral for depression with less cognitive effects versus bilateral for rapid response or catatonia), determines stimulus parameters, monitors seizure adequacy, manages post-procedural complications (particularly transient cognitive effects), and coordinates the treatment course (typically 6-12 sessions over 2-4 weeks for acute treatment).