ABSTRACT Group cohesion is considered a key group therapeutic factor; however, limited knowledge exists on the session-to-session development of group cohesion in the context of training groups or on the factors that may predict such developments. We utilized multilevel growth modeling to examine the longitudinal changes in 44 counselor trainees’ ratings of group cohesion in seven experiential growth groups across four semesters. We found significant between-person variability in the growth trajectory of group cohesion and identified trainee self-disclosure and gender as significant predictors of group cohesion. The results indicated the complexity of within- and between-person changes in group cohesion in training groups and the need for further research to identify potential predictors of these changes. We also discussed ways for group facilitators to attend to self-disclosure as an important marker of group cohesion.