Seasonal changes in behavior und physiology have been recognized in humans since ancient times. Their relevance to psychiatry was described in further detail at least as early as in the middle of the last century. However, it was only in the past six years that clinical studies were undertaken systematically to describe the profile of a syndrome that was called seasonal affective disorder (SAD) by Rosenthal et al. It emerged, that this condition is characterized by changes in affect with depressed mood, anxiety and irritability and decreased energy. In contrast to major affective disorder these patients exhibit increased appetite with carbohydrate craving, weight gain and an increased duration of sleep. Seasonality of mood and behavior appears to be a dimension affecting many different people including normal individuals and those with conditions other than SAD. In vulnerable individuals these changes may reach symptomatic levels whereas in normals they may be regarded as acceptable fluctuations.