Photopheresis or extracorporeal photochemotherapy (ECP) is a cellular therapy which combines a leukapheresis followed by ex vivo treatment using psoralen and ultraviolet A irradiation before reinfusion into the patient. Its mechanisms of action remain unclear and selective photodestruction of leukocytes cannot explain the long-lasting immunomodulatory effects. Recent studies demonstrated that ECP down regulates the immune response and induces tolerance through the maturation of dendritic cells and the production of regulatory T cells. Based on these effects, ECP is mainly used for treatment of Sezary syndrome, graft-versus-host disease, organ graft rejection and autoimmune diseases. However, it is still not clear how ECP both activates tumor immunity against cutaneous T-cell lymphoma and induces tolerance in autoreactive disorders. In addition, the use of adjuvant therapies, the long-term effects and various treatment protocols remain to be investigated along with the specific indications.