The need to develop validated outcome measures to assess response to therapies in single joints has been recognized. In 2004, a task force was established to assess established and novel outcome measures in accordance with the OMERACT filter (truth, discrimination, and feasibility) for single joint assessment. This report describes the proceedings of the single joint assessment special interest group (SIG) at OMERACT 9, including an updated literature review of imaging of the knee joints, with a focus on the extent to which these modalities fulfill the OMERACT filter. A series of studies are reported that examine patient reported, clinical examination, and imaging outcomes in therapeutic studies in knee arthritis. A summary of discussions from the meeting are presented that raise many of the ongoing challenges in establishing appropriate domains to evaluate a range of conditions and potential therapeutic interventions. Because of emerging drug candidates and modalities targeting individual joints, the ongoing work of this SIG is providing the evidence base that can be used to establish a core domain set to incorporate as outcomes in future studies. The Journal of Rheumatology Copyright © 2009. All rights reserved.