Constitutions are critical links between a foundational past and an aspirational future. This dual role opens the door to significant disharmonies both within the text and between the document and the people. There are times, however, when efforts to reconcile growing disharmonies in a constitutional system are no longer sufficient, requiring a new approach that will transform the identity of the constitutional order. The periodic need for transformative change has led scholars to analyze constitutional revolutions and their role in constitutional systems. This dissertation examines how the amendment power can be used to legitimately produce a constitutional revolution, altering the core identity of a constitutional system. First, I argue that revolutionary amendments can be classified along two dimensions: institutional and socio-normative. While institutional amendments restructure the delegation of sovereignty in the state, socio-normative amendments shift the constitution’s core values and redefine the demos. Drawing on civic republican theory, I then argue that the process of enacting revolutionary amendments must approximate the primary constituent power by fostering citizen representation and deliberation in both the drafting and ratification of the amendment, mirroring mechanisms that would be used to draft a new constitution. In this way, the amendment can make a claim to a new popular sovereignty independent of the existing document. This theory, which I call the Approximation Thesis, can help determine when a revolutionary amendment will be seen as a legitimate constitutional change by the citizens of the state. Chapter 2 lays out the theoretical framework by introducing the concept of the revolutionary amendment and offering a normative assessment of the path to legitimation, connecting the concept to the literature on constituent power and popular sovereignty. The subsequent chapters provide empirical support for this theory. In doing so, I conduct in-depth case studies of revolutionary constitutional change in Ireland (Chapter 3) and the United Kingdom (Chapter 4), focusing specifically on the 2018 repeal of Ireland’s Eighth Amendment and the 2016-2021 Brexit process. In Chapter 5, I conduct experimental analysis in the United States to test the impact of participatory amendment procedures on the legitimacy of constitutional change, both ordinary and revolutionary.