This article reports on a research synthesis of the relation between alliance and the outcomes of individual psychotherapy. Included were over 200 research reports based on 190 independent data sources, covering more than 14,000 treatments. Research involving 5 or more adult participants receiving genuine (as opposed to analogue) treatments, where the author(s) referred to one of the independent variables as “alliance,” “therapeutic alliance,” “helping alliance,” or “working alliance” were the inclusion criteria. All analyses were done using the assumptions of a random model. The overall aggregate relation between the alliance and treatment outcome (adjusted for sample size and non independence of outcome measures) was r = .275 (k = 190); the 95% confidence interval for this value was .25–.30. The statistical probability associated with the aggregated relation between alliance and outcome is p < .0001. The data collected for this meta-analysis were quite variable (heterogeneous). Potential variables such as assessment perspectives (client, therapist, observer), publication source, types of assessment methods and time of assessment were explored.