This chapter examines how transitional governance arrangements are constituted in peace and transitions processes. We map and analyse the relationship between the legal ‘constitutionalisation’ of the transition, and the political tasks and dilemmas of transition management. We frame the dilemmas of constituting transitional institutions and tasks, outlining the type of crisis which transitional governments are institutionalised to respond to. Based on a review of peace and transition arrangements between 1990 and 2015, we set out the five main options for constituting such transitions, and examine the distinctive transitional tasks and timelines established in these constitutional instruments. We argue that legal form is driven by the interaction between five factors, rather than any one fact. In conclusion we consider the ‘predicable unpredictability’ of transitional governance arrangements. We consider how they have needed to amended and adapted over time, and how the choice of legal form can both enable and limit capacity for adaptive transition management that is responsive to new political events. We suggest that the constitutionalisation of transitions should plan for ‘predictable unpredictability’ and the need for ‘adaptive management’ which will be likely to require new constitutional iterations over time.