This research project stems from a single puzzle: how can constitutional amendments be unconstitutional? Adopting a combination of theoretical and comparative enquiries, this thesis establishes the nature and scope of constitutional amendment powers by focusing on the question of substantive limitations on the amendment power, looking at both their prevalence in practice and the conceptual coherence of the very idea of limitations to amendment powers. The thesis is composed of three parts. The first part is comparative. It examines substantive explicit and implicit limitations on constitutional amendment powers through manifold descriptions of a similar constitutional phenomenon across countries, demonstrating a comprehensive pattern of a constitutional behaviour. This process is theory-driven, and the second part of the thesis constructs a general theory of unamendability, which explains the nature and scope of amendment powers. The third part explains how judicial review of amendments is to be conceived in light of the theory of unamendability, and further assesses the possible objections to the theory of unamendability. The theory of unamendability identifies and develops a middle ground between constituent power and pure constituted power, a middle ground that is suggested by the French literature on ‘derived constituent power’. Undergirding the discussion, therefore, is a simple yet fundamental distinction between primary constituent (constitution-making) power and secondary constituent (constitution-amending) power. This distinction, understood in terms of an act of delegation of powers, enables the construction of a theory of the limited (explicitly or implicitly) scope of secondary constituent powers. This distinction is supplemented by a further one, between various shades of secondary constituent powers along a ‘spectrum’, a theoretical construct that links amendment procedures to limitations on amendment powers. The theory of unamendability explicates the limited nature of amendment powers and the practice of judicial review of amendments, thus clarifying the puzzle of unconstitutional constitutional amendments.