Psoriatic arthritis (PsA), recognised for over 100 years, is common representing the second most frequent diagnostic category after RA and occurring in up to 10% of patients with skin psoriasis. The pathogenic connection between psoriasis and arthritis is not yet clear although our understanding of the mechanisms of disease has progressed significantly in recent years. Factors including immunogenetics, infection, autoimmunity, angiogenesis, trauma and the nervous system are implicated in the pathogenesis of PsA. Organ involvement is largely restricted to the connective tissue of the skin and joints, including both the synovial tissue and sites of entheseal attachment. This restricted inflammatory response suggests that either a common antigen driving the immune response or that antigenic proteins or cells are present at these sites only having migrated the or arising de novo. The epidemiology of Psoriasis has been extensively examined since the 1960's, however there have been few large epidemiological studies of PsA. In addition, the lack of diagnostic criteria for the diagnosis of PsA until 1973, and the diffuse clinical manifestations of this condition have hindered meaningful conclusion regarding the epidemiology of this form of arthritis.