Abstract: Psoriasis is a common chronic inflammatory disease of the skin affecting approximately 2% of Caucasians. Psoriasis has a worldwide distribution, with prevalence varying according to race and geographic location. Numerous population-, family- and twin-based studies point to a very strong genetic component of this disease. Psoriasis is a complex disease, as suggested by a very unclear and variable pattern of inheritance and a higher frequency in families of dizygotic twins than in those of monozygotic twins. So far 9 psoriasis susceptibility loci have been identified (PSORS1-9) but only three (PSORS1, PSORS2 and PSORS4) have been replicated in more than one study. The strongest genetic association has been found with the HLA-C region on the short arm of chromosome 6. Failure to reach 100% concordance in monozygotic twins points to a multifactorial aetiology of psoriasis where environmental factors play an important role in genetically predisposed individuals. Clinical, histological and ultrastructural evidence suggests that psoriasis is a T cellmediated disease where T cell activation is followed by release of pro-inflammatory cytokines, leukocytic infiltration of the skin, abnormal keratinocyte proliferation and angiogenesis. It is not known which exogenous or endogenous antigen(s) is responsible for triggering T cell activation or which genes play a fundamental role in psoriasis. Research is being carried out in an attempt to answer these questions. Here we review the main pathogenetic and epidemiological aspects of this skin condition.