The authors review the epidemiology, diagnosis and assessment, possible causes, and treatment of seasonal affective disorder (SAD). They present three new nonpharmacologic interventions that have been developed for patients with winter depression ó bright light therapy, dawn-dusk simulation therapy, and negative air ionization therapy ó and review results for antidepressant drugs. SAD is marked by recurrent major depressive episodes that begin in the fall or winter and remit in late winter or spring. Episode onset appears to depend on outdoor daylight availability (or the time of sunrise), and prevalence is higher at northerly latitudes where winter nights are longer. Patients tend to show atypical neurovegetative symptoms of hypersomnia and hyperphagia ó which are positive predictors of response to light therapy ó as well as pervasive daytime fatigue. Light therapy may act by inducing phase shifts of circadian rhythms (as marked by the timing of melatonin secretion), and may also enhance serotonin function. Post-awakening bright light exposure at 10,000 lux for 30 minutes daily provides an effective, efficient dosing regimen. Timing of light exposure may be critical, and the authors present a schedule based on the individual's habitual sleep pattern. Dawn simulation uses specialized apparatus to present a mimic of pre-sunrise springtime illumination to patients as they sleep, thus illuminating the bedroom when it is still dark outdoors in winter. High-density negative air ionization is a novel, nonphotic intervention which, although imperceptible, was effective in a placebo-controlled trial. Trials of specific serotonin reuptake inhibitors have shown moderate benefit for patients with SAD, but remission rates fell short of those for bright light therapy. The authors discuss potential applications of bright light therapy for non-SAD conditions including subsyndromal SAD, premenstrual depression, and bulimia nervosa. A doctor's office questionnaire, the Personal Inventory for Depression and SAD, is included to assist in prescreening patients for seasonal pattern.