Cohesion is the most popular relationship construct in the group therapy literature. This chapter reviews common definitions of cohesion and the most frequently studied measures. The authors also discuss a measure that may clarify group relations using two latent factors (quality and structure) to explain common variance among frequently used group relationship instruments. The results of a meta-analysis examining the relation between group cohesion and treatment outcome in 55 studies are presented. Results indicate that the weighted aggregate correlation between cohesion and treatment outcome was statistically significant r = .26, reflecting a moderate effect size (d = .56). Six moderator variables were found to significantly predict the magnitude of the cohesion–outcome association (type of outcome measure, leader interventions to increase cohesion, theoretical orientation, type of group, emphasis on group interaction, dose or number of group sessions). Patient contributions, diversity considerations, and evidence-based therapeutic practices are highlighted.