This study entails the use of meta-analytic techniques to calculate and analyze 18 independent and 52 nonindependent effect sizes across 17 published studies of psychological assessment as a therapeutic intervention. In this sample of studies, which involves 1,496 participants, a significant overall Cohen's d effect size of 0.423 (95% CI [0.321, 0.525]) was found, whereby 66% of treatment group means fell above the control and comparison group means. When categorical variables were taken into account, significant treatment group effects were found for therapy process variables (d = 1.117, [0.679, 1.555]), therapy outcomes (d = 0.367, [0.256, 0.478]), and combined process/outcome variables (d = 0.547, [0.193, 0.901]). These findings appear to be robust on the basis of fail-safe N calculations. Taken together, they suggest that psychological assessment procedures--when combined with personalized, collaborative, and highly involving test feedback--have positive, clinically meaningful effects on treatment, especially regarding treatment processes. They also have important implications for assessment practice, training, and policy making, as well as future research, which are discussed in the conclusion of the article. (Contains 2 figures, 2 tables and 1 footnote.)