Constitutional unamendability is a major topic of interest in comparative constitutional studies. It holds that constitutions possess a essential core or ‘basic structure’ which cannot be amended by that constitution’s designated amending power, and generally allows the judiciary to strike down constitutional amendments that violate this unamendable core. Unamendability arguably represents the pinnacle of constitutionalism – its high point as well as the ultimate limit of constitutions and judicial power. Because analysis of this topic forces us to consider the ultimate purpose of constitutions, it requires perspectives external to constitutional law. The purpose of this thesis is to provide a republican approach to constitutional unamendability. It considers the principles underpinning unamendability firstly at a theoretical level and then at a comparative constitutional level, comparing how unamendability is treated in the constitutions of Ireland, India, Germany, Brazil, and France – each of which approaches unamendability in a distinct way. It excavates salient republican principles on constitutional change and applies them to unamendability at both levels. The thesis argues that unamendability violates these republican principles, namely mutability, living constituent power, and contestability. Further, judicially-enforced unamendability has the effect of making the judiciary the ‘final say sovereign’ within the constitution, undermining republican defences of judicial review. The thesis concludes with some tentative suggestions on how republican constitutionalism can approach unamendability where it exists. In attempting to offer a republican approach to unamendability, this thesis does not deny that every constitution has an essential basic structure. However, it does deny that the definition and protection of this core is the exclusive province of the judiciary. In a republic, it is the responsibility of citizens to maintain a constitution’s basic structure, as well as to judge whether it is worth saving in the first place.