Psoriatic arthritis (PsA) is a chronic infl ammatory arthritis associated with psoriasis. It is a member of the spondyloarthritis (SpA) group and its most frequent subtype. It is diffi cult to determine the epidemiology of PsA due to the absence of universally accepted diagnostic criteria. Th e prevalence of PsA among patients with psoriasis is around 30% (varying from 6% to 42%). It is higher in Europe and the USA. In Asian countries, the prevalence of PsA among patients with psoriasis has been estimated to be less than 10%. Th ere is still a high prevalence of undiagnosed PsA in patients with psoriasis (up to 30%). Dermatologists should be encouraged to regularly screen their patients for PsA using several accepted screening tools. Prevalence estimates of PsA in Europe range from 0.05% in Turkey and the Czech Republic to 0.42% in Italy. Prevalence estimates of PsA in South America and Asia are lower than in Europe, and range from 0.02% in China to 0.07% in Argentina. Th e annual incidence of PsA in patients with psoriasis is 2 to 3%. Patients with severe psoriasis and nail changes are at greater risk of developing PsA. Th e fi rst classifi cation criteria for PsA were proposed by Moll and Wright in 1973. In 1991, the European Spondyloarthropathy Study Group (ESSG) criteria established for the fi rst time that PsA can also occur in the absence of psoriasis. Th e ASAS criteria for axial and peripheral SpA can be used in combination for the entire SpA population, including patients with PsA. Th e CASPAR criteria, developed by the Group for Research and Assessment of Psoriasis and PsA (GRAPA), have very high specifi city and sensitivity (98.7% and 91.4%, respectively). Although they are not diagnostic criteria, the CASPAR criteria have been accepted not only in research, but in clinical practice as well.