Accountability for past atrocity crimes and violations of human rights are part of the R2P agenda. Transitional justice procedures are assigned the task of bringing perpetrators to justice, thus redefining moral boundaries and deterring future violations. As such, transitional justice mechanisms - trials, truth commissions, but also amnesties - forge a link between the past, the present, and the future by developing appropriate legal frameworks and institutions that are legitimate and respect the rule of law in general. However, the potential of transitional justice mechanisms - and, in particular, trials - has been widely contested. Does accountability for past atrocities contribute to improving security and justice in post-conflict societies? Contemporary transitional justice operates in the aftermath of “multipolar” and “horizontal” violence, when both state and non-state actors are involved. Such complex conflicts pose major challenges to transitional justice procedures, and to the implementation of R2P processes. Based on a data set of 63 (post)conflict societies from 1976 to 2012, the impact of transitional justice mechanisms on security in terms of levels of different types of violence, rule of law, and general institutional capacity is analyzed for a period of up to ten years. Even in the wake of complex conflicts, transitional justice mechanisms in general do not have counterproductive or adverse outcomes. Improvement of security in terms of a reduction of violence is the most likely and consistent impact of transitional justice mechanisms. In conclusion, it is suggested where to focus resources and efforts in transitional justice in post-conflict countries in order to contribute to state resilience.