During the last decade Seasonal Affective Disorder (SAD) and light therapy have been the subject of many publications. Even though many questions still remain unanswered, there is reason to assume that light therapy is an effective treatment for winter depression. This article describes the history and development of Seasonal Affective Disorder and light therapy, gives an overview of the controlled studies and discusses methodological difficulties in SAD research. Current hypotheses are described and the position of light therapy within clinical practice is demonstrated.