Seasonal affective disorder is a mood disorder that is a subtype or qualifier of major depressive disorder or bipolar disorder in the Diagnostic and Statistical Manual of Mental Disorders. It is characterized by depressive symptoms that occur at a specific time of year (typically fall or winter) with full remission at other times of year (typically spring or summer). Possible risk factors include family history, female sex, living at a more northern latitude, and young adulthood (18 to 30 years of age). With the temporal nature of the mood episodes, diagnosis requires full remission when the specified season ends and two consecutive years of episodes in the same season. First-line therapy for seasonal affective disorder includes light therapy, antidepressants, and cognitive behavior therapy, alone or in combination. Commercial devices are available for administering light therapy or dawn simulation. The light intensity and duration of treatment depend on the device and the patient’s initial response, but 2,500 to 10,000 lux for 30 to 60 minutes at the same time every day is typically effective. Lifestyle interventions, such as increasing exercise and exposure to natural light, are also recommended. If seasonal affective disorder recurs, long-term treatment or preventive intervention is typically indicated, and bupropion appears to have the strongest evidence supporting long-term use. Continuing light therapy or other antidepressants is likely beneficial, although evidence is inconclusive. Evidence is also inconclusive for psychotherapy and vitamin D supplementation.