The efficacy of phototherapy as a potential treatment modality for certain types of depression is examined. Phototherapy is suggested to be useful in relieving depressive symptoms in individuals exhibiting circadian rhythm disturbances. Particular attention is paid to the characteristics of endogenous and seasonal depressives. Endogenous depressives exhibit phase-advances in the sleep-wake cycle, REM-sleep, temperature, cortisol, and melatonin rhythms. Preliminary evidence indicates phase-delays in the rhythms of seasonal depressives. A possible physiological pathway through which light may act upon circadian rhythm generators to correct rhythm abnormalities is discussed. Theoretical positions which have been proposed to explain the underlying mechanisms of phototherapy are evaluated. Research findings are discussed within these frameworks. Major experimental weaknesses limit the usefulness of many research findings as support for the effectiveness of phototherapy in treating depression. An attempt is made to clarify some of these problems and to highlight the methodological shortcomings of research in this area. Strategies for future research methodology are recommended. Better experimental design which controls for placebo effects is needed in order to properly evaluate the antidepressant effects of phototherapy.