The use of bright light (phototherapy) for psychiatric disorders has recently generated much interest among researchers and the lay population. The authors review the treatment studies of phototherapy for seasonal and non-seasonal depressive disorders, and the empirical evidence for theories of the psychophysiology of phototherapy. Although its mechanism of action remains to be explained, phototherapy appears to be a safe and effective treatment for seasonal depression and a promising treatment for non-seasonal depression. Further questions and future research directions are presented.