A relatively small group size is the primary characteristic of contemporary terrorist organizations. Nonetheless, a division of labor occurs among terrorists, and some structure is needed to channel communication and coordinate activities. The small size of terrorist groups imposes on them an infrastructure quite different from that of underground movements during World War II. In contrast to a hierarchical organization where the leader is at the top, the leader in a centrifugal infrastructure is at the center. Leaders of terrorist organizations, by virtue of being in the center of their organizations, act as direct catalysts on action by planning and giving orders, and they also participate in the action. The centrifugal system secures more direct and rapid communication and is independent of society's support. Direct access to the leader by the rank and file allows the latter to identify with the power of mythical qualities usually attributed to the leader. The leader is also directly involved in the selection and recruitment of new group members. Centrifugal groups tend to seek cooperation and allies, not with stronger groups but with those whom they can easily manipulate. When either centrifugal or hierarchical organizations are threatened by stresses from within, their respective leaders will use external violence to keep members together. The development of appropriate and effective counterterrorist policies is examined. 15 references and 2 figures