This article reflects upon Rex Stockton's contributions to the area of group process and outcome research. Through a series of research studies and literature reviews, Stockton has contributed to the current understanding of the precise mechanisms of change through which groups are effective. His work in the areas of structure, cohesion, premature termination, and member goals and expectations is reviewed. In addition, his emphasis on utilizing this information to support the work of practitioners is highlighted. Stockton's contributions to the group process and outcome literature are examined in relation to current research in the area and to recent efforts to develop and disseminate empirically supported principles to guide the work of practitioners.