Outcome assessment in osteoarthritis (OA) clinical trials requires the use of valid, reliable, and responsive measurement techniques. Clinical assessment procedures examine patient relevant issues, while imaging procedures and biologic marker assays probe different aspects of the underlying disease process. At present, there is no international agreement on which standardized procedures should be used in future studies or which domains should be included. Nevertheless, significant progress has been made in recent years toward attaining that goal and there are numerous measurement procedures, differing in their conceptual and clinimetric properties, from which to select a core set. The following review provides a description of candidate variables that merit consideration in reaching international harmonization on outcome measures for future OA clinical trials.