Collaborative assessment methods (CAMs) involve working with clients during all phases of the assessment process, from goal definition to interpretation of the testing results to the recommendations and conclusions. In this article, we define CAMs, provide clinical examples, and then meta-analyze the published literature to assess their effectiveness on distal treatment outcomes. Our meta-analytic results indicate that CAMs have positive effects on three outcome domains: a moderate effect on treatment processes, a small-to-moderate effect on personal growth, and a small effect on symptom reduction. There is little research evidence on the immediate, in-session effects of CAMs. We include diversity considerations, training implications. and therapeutic practices grounded in this research evidence. (PsycInfo Database Record (c) 2023 APA, all rights reserved).