Psoriatic arthritis (PsA) is a condition characterised by inflammatory arthritis and enthesitis, which can occur in nearly a third of patients with psoriasis. These patients share unique genetic factors and immunopathogenic features in the joints, enthesium, and skin. The clinical characteristics of the joint disease associated with psoriasis have been more clearly elucidated over the past 30 years. Adding to this is an appreciation that standard systemic agents for psoriasis, such as methotrexate and ciclosporin, are also effective for PsA. However, it has been the recognition of the immunological features and the emergence of “biologicals” that target specific immune/inflammatory pathways of selected disease processes that have generated the current excitement and interest in autoimmune disease. These developments have given us new insights into pathogenesis while bringing new, more effective, and safer treatments for both the skin and joints. This supplement of the Annals of the Rheumatic Diseases is a compilation of articles summarising the state of the art in this field.

Several centres have contributed the majority of what we know about PsA due to longstanding research interest. Numerous inadequately explored questions remain about disease classification, disease pathophysiology, and the development of validated methodologies to assess disease outcomes, both in longitudinal cohorts and in clinical trials. There is an increasing international exchange of ideas among dermatologists and rheumatologists and between these two specialties about emerging findings in psoriasis and its related arthritis. This exchange has been broadened by advances in related fields in rheumatology, such as the other spondyloarthropathies and rheumatoid arthritis, and in dermatology, such as T cell mediated skin diseases. It has been supported by the biopharmaceutical industry, which sponsors both basic and applied research. Also participating …