Phototherapy, whose first application for psoriasis dates back to almost a century now, is still an actual mainstay of treatment. We discuss in detail the radiophysical aspects involved in the therapy, the different treatment modalities, and all aspects related to clinical application of phototherapy. By looking at new insights on the molecular mode of action, it becomes evident that phototherapy is in fact the oldest "biological" therapeutic strategy, whose target is directly the T-cell-mediated immunopathology of psoriasis. In an outlook, we discuss finally the current cost effectiveness calculations, important issues in times of increasingly tight public health budgets. In summary, this review points out that phototherapy is clearly a first-line therapy that is safe and effective. Guidelines in the patient management still have to be harmonized, however, and further trials to improve the fine tuning of irradiation protocols are still necessary.