Cohesion is the most popular of several relationship constructs in the clinical and empirical group therapy literature. This article reviews the most frequently cited definitions and studied measures of group cohesion. We briefly introduce a new measure, the Group Questionnaire, which elucidates group relationships by suggesting two latent factors of cohesion-relationship quality (positive bond, positive work, and negative relationship) and structure factors (member-leader and member-member). To further understand the literature, we conducted a meta-analysis examining the relationship between cohesion and treatment outcome in 40 studies. Results indicate cohesion that the weighted aggregate correlation was statistically significant with outcome r = .25, k (40), N (3,323), z = 6.54 (p < .05) with a 95% confidence interval of .17 to .32. In addition, five moderator variables were found to significantly predict the magnitude of the cohesion outcome correlation (age, theoretical orientation, length, and size of group, as well as interventions intended to enhance cohesion). Consideration of measures and practices to improve treatment outcome are highlighted.