The constitution in the material sense is not particularly popular among lawyers. As its conceptual predecessor (the doctrine of the historical constitution), the constitution in the material sense points toward a series of non-specified norms relating to the basic structure of government and to the relationship between citizens and the state. As such, it lacks the certainty often associated with legal rules. In this chapter, we will focus on two judgements of Latin American constitutional courts where, by relying on a specific conception of the material constitution, judges sought both to define the competence of the amending authority and to challenge the judicial imposition of implicit limits to the power of constitutional reform.