Objective: Alliance is considered to be one of the oldest, most well-researched themes in psychotherapy. This article presents a brief history of alliance, its most common definitions and measures used in group treatment, and a meta-analytic review of the alliance-outcome relationship in group therapy. Method: Major databases were searched for articles published between 1969 and 2019. We conducted a random effects meta-analysis of published articles that reported the relation between alliance and treatment outcome within the group format. Results: Results from 29 studies including 3,628 patients indicate that the weighted average correlation between alliance and treatment outcome is significant r = .17 (p d = 0.34). Heterogeneity of effect sizes was low (Q = 36.47, df = 28, p = .131, I2 = 23.2). Two variables were found to moderate the alliance-outcome association (treatment orientation and reporting perspective of alliance). Conclusion: With a substantial link between alliance and outcome established, the importance of therapeutic alliance rupture and repair in group therapy is discussed and treatment guidelines are outlined. (PsycInfo Database Record (c) 2021 APA, all rights reserved)