Seasonal affective disorder is a syndrome of classical depressive symptoms such as reduced energy, initiative and mood combined with atypical symptoms of increased appetite, weight and sleep duration. The symptoms recur each winter and disappear again in spring or early summer. The prevalence ranges from 1% to 10% in Scandinavian populations. Reduced light exposure, melatonergic and serotonergic disturbances are suggested pathogenetic factors. Light therapy offers convincing effect with minimal adverse effects and remains first-line treatment along with selective serotonin reuptake inhibitors.