The alliance continues to be one of the most investigated variables related to success in psychotherapy irrespective of theoretical orientation. We define and illustrate the alliance (also conceptualized as therapeutic alliance, helping alliance, or working alliance) and then present a meta-analysis of 295 independent studies that covered more than 30,000 patients (published between 1978 and 2017) for face-to-face and Internet-based psychotherapy. The relation of the alliance and treatment outcome was investigated using a three-level meta-analysis with random-effects restricted maximum-likelihood estimators. The overall alliance–outcome association for face-to-face psychotherapy was r = .278 (95% confidence intervals [.256, .299], p < .0001; equivalent of d = .579). There was heterogeneity among the effect sizes, and 2% of the 295 effect sizes indicated negative correlations. The correlation for Internet-based psychotherapy was approximately the same (viz., r = .275, k = 23). These results confirm the robustness of the positive relation between the alliance and outcome. This relation remains consistent across assessor perspectives, alliance and outcome measures, treatment approaches, patient characteristics, and countries. The article concludes with causality considerations, research limitations, diversity considerations, and therapeutic practices.