This article conceptualizes the phases of group development using Y. M. Agazarian's theory of living human systems (1997) and K. Lewin's field theory (1951). Linking these theories to operational models builds a bridge to research by making it possible to generate specific hypotheses. The basic systems-centered hypothesis can then be tested empirically: that weakening the restraining forces at the boundary between each subphase of system development releases the driving forces inherent in all living human systems, so that the system moves toward the next phase of development-in the direction of the inherent system goals of survival, development, and transformation. The discussion focuses on the implications for group leaders in organizations and therapeutic practice.