What is the autonomy of EU law, and why does that matter?

This article argues that the autonomy of EU law conveys a set of rules and prin- ciples but also constitutes a principle of EU law on its own terms. Its features suggest a distinctive existential character, in light of its internal and external reach but, particularly, a quality of extremity that has come to define its impli- cations. Reflecting on the nature of autonomy matters because of the closing down of space for compromise it produces and what is uncovered about the nature of EU primary law in consequence. The effects of autonomy have prin- cipally concerned the jurisdiction of the Court of Justice, but the wider focus on autonomy of ‘Union decision-making’ as a ‘core principle’ of Brexit negotia- tions, for the EU part, reactivates a more generalised understanding of what the principle commands. That process also tests the extent to which an under- standing of autonomy as an existential principle should be sustained.