This chapter examines the puzzle of the extraordinary resilience shown by a wide array of armed groups in cities across the world, and particularly in the Global South. While some authors had forecasted the advent of ‘feral cities’ where informal settlements dominate, the current state of affairs is much more complex. While cities have shown the ability to defuse conflict to a degree, disruptive violent actors are indeed present in several urban areas. These urban armed groups have often managed to resist state repression for decades and even thrive in the meantime. This chapter investigates the multiple facets of this apparent resilience. It identifies the aspects that the rest of the book focuses on—namely: territories, relations, and members—and thus the academic debates that this work contributes to. The chapter then presents the book’s main protagonists—Rio de Janeiro’s gangs, factions, and police forces—as well as the types of sources used in its analysis.