Over the last three decades, a growing number of countries have experienced a transition from authoritarianism to democracy, and the new governments have been increasingly expected to address past human rights violations. While the academic literature on the impact of human rights prosecution is relatively well developed, the literature on the causes of such prosecution is still sparse. Why do states pursue criminal prosecutions against former state officials on the charge of human rights violations? This article answers this question by testing three key theories: the balance of power between old and new elites, transnational advocacy networks, and the diffusion theory. I conduct a crossnational study of 71 countries that were in a state of democratic transitions between 1980 and 2006, using a new dataset on domestic human rights prosecutions. I find strong evidence to support the transnational advocacy networks and diffusion explanations. First, active domestic and international human rights advocacy for individual criminal accountability is a key factor guaranteeing persistent and frequent human rights prosecutions. My study further shows that domestic advocacy plays a crucial role in criminal prosecutions of high-profile state officials while international pressure is more effective in promoting prosecutions of low-profile officials. Second, the diffusion theory is also supported since the occurrence of human rights prosecution in neighboring countries is a relevant factor. Interestingly, transitional countries are most sensitive to trials occurring in culturally or linguistically similar countries and this supports the constructivist norm diffusion theory, which focuses on the role of identity and communication in the diffusion process. However, I find that the power balance explanation, which has been the prevailing explanation, is valid only for the immediate use of human rights prosecutions.