This article reviews the current state of comparative genocide theorizing, focusing on theories that attempt to account for the causes of genocide and the processes of genocidal killing. The literature is divided into three broad categories, based on the relative weight given to (a) individual or group agency, (b) structural factors, or (c) processes of identity construction in accounting for the origins and unfolding of genocidal destruction. The discussion of agency-oriented approaches focuses on theories that suggest that genocide is driven, in terms of decision making and perpetration, by elite decision makers, front-line perpetrators, and societal behavior. The literature on structural approaches is broken down into theories that stress the importance of culture, institutional organizations, societal cleavages, structural crises, regime type, modernity, and ideology. The final section reviews the literature on processes of collective identity construction. The article suggests throughout and in conclusion that although comparative genocide theorizing has come a long way in proposing a number of different explanations for the onset of genocide and the nature of genocidal processes, more work needs to be done with respect to the precise operationalization and testing of theories according to more rigorous comparative methodological practices.