This note examines the UK Supreme Court's judgment in the Brexit case, Miller v Secretary of State for Exiting the European Union. The case upheld the decision of the High Court, which rejected the claim that the foreign affairs prerogative provided a legal basis for giving notice to EU institutions of the UK's intention to withdraw from the EU. But the Supreme Court's preferred basis for dismissing that claim rested on the more general proposition that significant constitutional change can only be effected by statute. This position offers the germs of a jurisprudence of constitutional change and was substantiated by means of an analysis of Parliament's dual capacity as legislator and constituent agent. Miller also includes important and potentially innovative dicta on the relationship between international and domestic sources of law.