Psoriatic arthritis occurs in 5 - 42% of patients with psoriasis. It is an inflammatory arthritis distinct from rheumatoid, being usually sero-negative, asymmetrical and often affecting the spine, sacro-iliac and distal interphalangeal joints. It runs a very variable course, from a mild non-destructive disease to a severe rapidly progressive erosive arthropathy, producing an 'arthritis mutilans' with a combination of bone lysis and joint ankylosis. Its pathogenesis is not as well understood as rheumatoid arthritis, but is thought to be similarly immune driven, with a qualitatively similar immunomodulatory cascade and cytokine profile. Quantitatively, however, there are distinct differences in cell ratios and cytokine levels that may well impact on therapeutic strategies. Current therapies, such as methotrexate and sulphasalazine, have yet to be shown to be significantly more effective than placebo in delaying damage and produce only marginal improvements in symptoms. The newer specific biological agents, such as the anticytokine antibodies, interleukins and more specific anti-T-cell therapies, are starting to be studied in psoriatic arthritis. The rationale for their use comes mostly from extrapolation of their efficacy in rheumatoid arthritis. It has yet to be seen whether they will be efficacious in treating the osteolysis, fibrosis and new bone formation particular to psoriatic arthritis. Any treatment for the arthritis must also help the skin. Greater understanding of psoriatic arthritis, its pathogenesis and natural history is required if we are to target these exciting but expensive therapies effectively.