How do we conceptualize the fragmentation of internally divided movements? And how does variation in fragmentation affect the probability and patterns of infighting? The internal politics of non-state groups have received increasing attention, with recent research demonstrating the importance of cohesion and fragmentation for understanding conflict dynamics. Yet there is little consensus on how to conceptualize fragmentation, the concept at the center of this agenda, with authors using different definitions and measures. In this paper we conceptualize fragmentation along three constitutive dimensions: the number of organizations in the movement; the degree of institutionalization across these organizations; and the distribution of power among them. We then show how variation across these dimensions can explain variation in important conflict processes, focusing on infighting.