The definition of axial disease in psoriatic arthritis has varied from isolated unilateral grade 2 sacroiliitis to criteria similar to those used for ankylosing spondylitis. Depending on the definition used, the prevalence of axial disease varies from 25% to 70% of patients with psoriatic arthritis. This article reviews the prevalence, clinical and radiologic features, pathogenesis, prognosis, and treatment of psoriatic spondylitis.