Phototherapy has been largely empirical. Thus, despite the long term use its mechanisms of action are quite unclear. Because of the recent achievements in photoimmunology and molecular photobiology we are now beginning to understand some of the mechanisms. Many of the effects are certainly mediated via induction of apoptotic cell death. Another major mechanism is the induction of immunosuppression. This review primarily focuses on recent advances in photoimmunology which will contribute to the further understanding how phototherapy acts.