Genocide and other crimes against humanity are not solely individual crimes. They possess a particular institutional nature that can be difficult for law to capture. Transitional justice relies on the institution of law as a companion to the political goal of enabling political transition and effecting justice in the wake of state crime. There are high expectations of law at such times, that it provide frameworks for peaceful transition, and assist in reconstruction and reconciliation. Yet law is poorly equipped to play such a role. Law's historical preoccupation is with accountability for particular individual crimes, whereas transitional justice, as a framework and practice, is focused on a broader program of institutional and social reconstruction. In this article, I suggest that if law is to be a companion to the political project of state transition, then we need to be developing new political-legal concepts to make law effective in these tasks. While we are developing new legal and political-legal bodies to deal with these crimes, such as truth commissions and international tribunals, we are developing few new legal concepts. If law is to deliver accountability for mass harms and partner transitional justice in reshaping and reconstructing a society in the wake of conflict, then new concepts and frameworks to support law in this task are necessary.