Over the last three decades, thousands of prosecutions for human rights abuses have progressed through domestic courts, a puzzling fact considering that state leaders have little incentive to punish their own agents. Previous studies have advanced rational-choice or sociological-institutionalist accounts of this phenomenon, emphasizing the role of political coalitions or regional cultures. Few, though, have recognized the local, private struggles that lie at the root of the trend toward domestic human rights enforcement. In this article, we develop a historical-institutionalist theory of normative change centered on the notion of "prosecutorial momentum." We contend that the rise in domestic trials against rights-abusing state agents in Europe and Latin America results in large part from the cumulative efforts of victims and human rights lawyers utilizing their rights to private criminal prosecution. Using a new data set and mixed methods, we offer a systematic analysis of how rights to private criminal prosecution, when activated in response to a legacy of repression, helps set in motion sustained efforts to pursue domestic enforcement and compliance with international law.National leaders have little rational incentive to punish the very agents on whom they rely for coercion. Yet increasingly often, domestic courts across the world bring state agents to trial for human rights violations. Between 1970 and 2010, for example, more than 3,000 domestic human rights prosecutions were initiated, and this has resulted in moderate improvements to physical integrity rights protections (Kim and Sikkink 2010:956-58). 1 This is evidence of the global but decentralized enforcement of international law and norms (Sikkink 2011). How can we account for this rise in domestic efforts for criminal accountability of human rights violations?While the human rights and transitional justice literature points to numerous explanations, we build on recent research that highlights domestic institutions as an explanatory factor behind the rise of human rights accountability efforts. Top-down rationalist or sociological explanations attribute the rise of prosecutions either to the stable preferences of political coalitions or to static regional cultures. Instead, we advance an historical-institutionalist argument about gradual change (Pierson 2004; Mahoney and Thelen 2010). Specifically, we contend that the rise in domestic trials against rights-abusing state agents results in large part from victims and human rights lawyers' litigating at the domestic level. By creatively utilizing existing legal-institutional tools over time, these opportunistic actors produce sustained campaigns for human rights accountability.We are not the first to make the argument that individuals interacting with available institutions can promote human rights enforcement (Collins 2010;Sikkink 2011;Burt 2013;Michel and Sikkink 2013;Davis 2014;Gonzalez Ocantos 2014). In particular, Michel and Sikkink (2013) introduce the importance of victims' rights in criminal proce...