This study of the complete life-spans of eight naturally-ocurring teams began with the unexpected finding that several project groups, studied for another purpose, did not accomplish their work by progressing gradually through a universal series of stages, as traditional group development models would predict. Instead, teams progressed in a pattern of “punctuated equilibrium,” through alternating inertia and revolution in the behaviors and themes through which they approached their work. The findings also suggested that groups' progress was triggered more by members' awareness of time and deadlines than by completion of an absolute amount of work in a specific developmental stage. The paper proposes a new model of group development that encompasses the timing and mechanisms of change as well as groups' dynamic relations with their contexts. Implications for theory, research, and practice are drawn.