Psoriatic arthritis was first described in the early part of the nineteenth century. Over the past 50 years, concepts of the disease have evolved as a result of clinical, epidemiological, radiological and immunogenetic study. Epidemiological and clinical investigations suggest that the disease is a unique arthropathy rather than the coincident occurrence of two common diseases. There are no validated criteria for classification; this is partly because of the heterogeneous clinical features associated with the disease, and the relapsing and remitting nature of both psoriasis and arthritis. Clinical subgroups have been proposed and have proved useful in study of the disease; however, there are inconsistencies and overlaps in the published data. The population prevalence of psoriatic arthritis is in the range of 2-10 per 10,000 although this is probably an underestimate as those with sacroiliac involvement only are not included. There are currently no incidence figures from population samples. The disease is slightly more common in females than males, although there is variation in the sex ratio by disease subgroup. There is evidence that hormonal and environmental factors play a role in the occurrence of disease.