The inadequate nature of research into group psychotherapy stems from the lack of a satisfactory general theory that accounts for the group as a social system as well as for the functioning of the individual. A critique is presented of the group-as-whole tradition and a brief introduction given to social system concepts, particularly as they relate to group developmental stages. The idea that groups progress through a series of stages implies the notion of the group as a single entity with its own organizational structure. Specific stages are described through which the group acquires interactional complexity. These are conceptualized as epigenetic phenomena in that the mature structure is attained through an invariate sequence of transformations. Adequate resolution of any one stage is a prerequisite for further development. Group developmental tasks are described for the following stages: Engagement, Differentiation, Individuation, Intimacy, Mutuality and Termination. This set of higher order concepts has been found useful as a theoretical background against which specific group events can be evaluated. By using the idea of stages, the leader can monitor the progress of the group as a unit and compare the activity of any one member with the collectivity.