Under what conditions do rebels succeed in establishing functional institutions in state-dominated areas? Canonical theories of rebel governance and state formation insist that territorial control is a necessary precondition for the development of governing institutions. Yet despite growing recognition that this claim is empirically incorrect and theoretically limiting, we lack knowledge about the conditions under which rebels succeed in governing civilians in areas where the state dominates. We argue that low state governance responsiveness towards rebel constituencies enables insurgents to overcome the challenges associated with establishing institutions in state-dominated areas. Low state responsiveness increases popular demand for insurgent institutions, decreases the costs associated with governing, and enables insurgents to collude with civilians to hide their institutions. Case studies from Ireland, South Africa, and Algeria illustrate our propositions. Our findings deepen knowledge on how rebels govern and expand their territorial reach, and shed light on alternative trajectories of state formation.