Psoriatic Arthritis (PsA) is a common condition that significantly impacts affected patients. The introduction of novel therapeutic agents for PsA has generated considerable interest in both clinical trials and in clinical care. Thus, there is a great need for standardized outcome measures to assess the activity of disease and the response to therapy. Because psoriasis is a heterogeneous and multi-faceted condition, defining outcome measures has been a challenge. To date, such measures have largely been adapted from related diseases, as described in this essay. Further research is needed to further develop outcome measures for PsA to facilitate optimal treatment of patients with PsA.