Psoriatic arthritis (PsA) is a common form of arthritis, accounting for approximately 15% of patients attending early synovitis clinics, the second most common diagnostic category after rheumatoid arthritis. PsA patients exhibit restricted organ involvement largely confined to skin, synovial tissue and sites of entheseal attachment. This restricted involvement in an inflammatory response suggests that either a common antigen/autoantigen is present at these sites which is driving the immune response or that there are shared proteins or cells which migrate to or are resident at these sites only. This chapter explores the evidence for common pathogenic mechanisms in the skin and joint in PsA. Further, the concept of similar mechanisms operating in the various subtypes of PsA is also explored. While evidence presented supports the concept of PsA being one disease with diverse clinical manifestations, further detailed genetic and mechanistic studies are required before a firm conclusion can be arrived at.