Whereas psoriasis is well defined as a clinical entity for more than a century, the pathogenesis of psoriasis is still open to debate, despite demonstration of multiple abnormalities in cell-kinetics, histomorphology and biochemistry. Recent research indicates altered immune function in psoriatic skin, which may explain some of the phenomena typical for this disease. The role of immunological pathomechanisms in psoriasis is further substantiated by the well-known success of immunosuppressive treatments. However, as long as putative auto-antigens, which are presumed to perpetuate the pathological immune reactions, are not identified, psoriasis cannot be classified as an autoimmune disease.