Psoriatic arthritis (PsA), an inflammatory arthritis associated with psoriasis usually seronegative for rheumatoid factor, has emerged as a more common and severe disease than previously appreciated. The disease is multifaceted. Thus the assessment of PsA requires attention to peripheral joint involvement, axial disease, dactylitis, and enthesitis, as well as the skin manifestations. In addition, the assessment of patient reported features such as patient assessment of disease activity, pain, fatigue, quality of life, and the new concept of participation are important. The assessment of damage and the assessment of tissue histology are also important outcome measures. This article summarizes these features of PsA as well as current knowledge on the instruments available for the assessment of these domains.