The terrorist organizations that have posed the greatest threat to international security are those with allies. Terrorist groups at the core of alliance networks, particularly the Islamic State and al Qaeda, define the threat today, as they are able to accrue and disperse the benefits of their alliances—including greater lethality, longevity, and resilience—to their partners. While the consequences of these alliances are clear, their causes remain poorly understood, especially with respect to why terrorist alliances cluster around a small number of organizations. I propose that groups ally with the organization at the core of a network to address organizational deficits. In addition, the prospective partners must have both complementary needs and the ability to link their ideologies and frames to build a shared identity. Finally, groups must overcome their inherent suspicions and build trust to ally. These three mechanisms lead to alliance formation, but they also offer numerous avenues for disruption.