Seasonal affective disorder (SAD) is a recurrent depressive disorder in which episodes occur at a particular time of year. The most frequent variant is winter SAD in which patients become depressed during the autumn or early winter, with remission or conversion to hypomania the following spring. Recognition of winter SAD is important because the diagnosis predicts a good response to the novel treatment of phototherapy. One other variant, summer SAD, has also been described and we will discuss this condition separately. Otherwise, we will confine our discussion to the winter variant which we will refer to as SAD.