A crucial and often misunderstood aspect of the study of terrorism is the organizational structure of groups. What affects the organizational structure of terrorist groups? How does the structure of a group affect its behavior? This chapter seeks to address these questions and argues for the importance of understanding organizational structure as a critical tool in counterterrorism analysis. Terrorist groups strive to balance efficiency with their need for security. The chapter provides a brief review of the previous research related to terrorist group organizational structures. It introduces several organizational structures and details some general trends of how organizational structure is affected by the group's environment and how structure, in turn, affects the group's behavior. The chapter explains the internal and external forces affecting terrorist organizational structure. When examining terrorist group organizational structures, both the internal and the external forces acting on the group have an effect on a group's behaviour.