Photopheresis is a form of phototherapy where specialized equipment is used to isolate a leukocyte fraction from the peripheral blood which is then exposed to photoactivated 8-methoxypsoralen and reinfused into the patient. At the time of its invention the treatment was conceptually based on the hypothesis of T cell vaccination, i.e. the observation in experimental studies that exposure of the immune system to physically modified T cell clones leads to a specific inhibition of T cell mediated autoimmunity. Consequently, photopheresis has been tried in a variety of conditions where T cells are thought to have a critical role and has shown clinical efficacy mainly in variants of cutaneous T cell lymphomas, graft-versus-host disease, systemic sclerosis, in solid organ transplant rejection and Crohn's disease. Evidence has accumulated that alterations in antigen presentation and the generation of regulatory T cells are induced by photopheresis and might be related to the observed clinical effects. Summarizing what has been published in the 25 years since its introduction into the clinic, photopheresis to date has found its place in the treatment of the above mentioned conditions as a well tolerated treatment option that can safely be combined with other established modalities. It can be expected that further research will help refine its clinical indications and close the gaps that still exist in our knowledge on when, how, and why photopheresis works.