Psoriatic arthritis is an inflammatory joint disease that is heterogeneous in presentation and clinical course. Evidence that this disease is distinct from rheumatoid arthritis and other spondyloarthropathies is based on data derived from characteristic clinical features, histopathologic analyses, immunogenetic associations and musculoskeletal imaging. Emphasis has centered previously on a dominant role for the T lymphocyte in the inflammatory process; however, studies provide support for a major contribution from monocyte-macrophages in the initiation and perpetuation of joint and skin inflammation. The occurrence of arthritis in the absence of psoriasis in a minority of patients with psoriatic arthritis, coupled with divergent genetic risk factors, indicates that psoriatic arthritis is distinct from psoriatic skin inflammation. A new terminology, psoriatic disease, has emerged that encompasses the various manifestations of tissue and organ involvement observed in many psoriasis patients, including inflammation in the joint, eye and gut. Moreover, adverse cardiovascular and metabolic outcomes in patients with psoriasis or psoriatic arthritis might be directly linked to the cutaneous and musculoskeletal manifestations of these diseases via subsets of circulating monocytes and tissue macrophages activated by inflammatory cytokine networks that arise in the skin and possibly the joint.