This article examines whether there are any limitations on constitutional amendment powers that are external to the constitutional system and above it—‘supra-constitutional’ limits. It considers the theory and practice of the relationship between natural law, international law or other supranational law, and domestic constitutional law in a comparative prism. After considering the alleged supremacy of supranational law over constitutional amendments, the author explores the problem of the relationship between the different legal orders in the external/internal juridical spheres, and the important potential and actual role of national courts in ‘domesticating’ supranational law and enforcing its supremacy. It is claimed that despite the growing influence of supranational law, state practice demonstrates that constitutional law is still generally superior to international law, and even when the normative hierarchical superiority of supranational law is recognized within the domestic legal order, this supremacy derives not from supranational law as a separate legal order, but rather from the constitution itself. Therefore, it is claimed that existing practice regarding arguments of ‘supra-constitutional’ limitations are better described by explicit or implicit limitations within the constitution itself, through which supranational standards can be infused to serve as valid limitations on constitutional amendment powers.