This article reports a systematic review of engagement measures for psychosocial therapy. MEDLINE, EMBASE, and PsycINFO databases were searched to identify English-language studies (published 1980 to February 2010) that reported on an instrument/rating scale to measure engagement in psychosocial treatment for mental health difficulties. Forty-seven studies were identified, reporting information on 40 measures of treatment engagement. Although our findings suggest that therapeutic engagement appears to be considered an important construct to assess, they also reveal that there is little consensus in the definition of engagement employed. Few measures are generalizable across treatment settings and clinical populations, and limited information is reported on the indices of reliability and validity. It is concluded that further work is required to develop adequate measures of therapeutic engagement.