In the youth treatment literature, the alliance has been defined and measured as a consensual or collaborative bond. In this article, we review varied definitions of the alliance, enumerate its frequent measures, and present clinical examples. We provide a meta-analytic review on the relation between the therapeutic alliance and treatment outcome in child and adolescent psychotherapy. In particular, this review only includes prospective studies of youth therapy that used an explicit measure of alliance. The meta-analysis of 28 studies revealed a weighted random effect size of r = .19 (k = 28, N = 2419, p < .01, 95% confidence interval [.13, .25]), which is a small to medium effect (equivalent to d = 0.39) consistent with the adult alliance literature and with prior youth meta-analyses. Given that a medium-large amount of heterogeneity was observed in effect sizes (I2 = 64.19%), theory- and method-based moderators were examined. Multiple moderators of the alliance-outcome association were found, including diagnosis class, type of therapy, study design (randomized controlled trials [RCT] vs. nonrandomized trials [non-RCT]), and treatment setting (inpatient vs. outpatient). Research limitations, patient contributions, and diversity considerations follow. The article concludes with research-informed practices for building and maintaining the therapeutic alliance with youth. (PsycINFO Database Record (c) 2018 APA, all rights reserved).