On 2 October 2016, in the so-called “Plebiscite for Peace,” the majority of voters rejected the peace agreement that the Colombian Government had negotiated with the Revolutionary Armed Forces of Colombia. The Colombian Government’s implementation of the agreement triggered review of the agreement by the Constitutional Court. This chapter aims to answer two questions arising from this case. First, can plebiscites—or referendums—confer legitimacy on the implementation of peace agreements? Second, can constitutional review of the implementation of a peace agreement confer legitimacy to that process? Correspondingly, this chapter grounds the following two claims. First, plebiscites—or referendums—can confer legitimacy to a peace agreement only where those mechanisms culminate in a fair and inclusive deliberative process. If it is likely to achieve those conditions, institutional designers should avoid scheduling this kind of participatory strategy within a transitional justice process. Second, instead—or additionally—institutional designers should employ mechanisms of constitutional review of norms implementing peace agreements. Constitutional Courts can ensure that peace agreements advance the entrenchment of democratic constitutionalism. Naturally, this can only be the case if Constitutional Courts are impartial and independent.