This article discusses how the EU legal order deals with cases where Member States deem that the principle of necessity justifies a deviation from EU law. It analyses the various exceptional clauses laid down in primary and secondary EU law, and discusses the tensions which their application may raise in the context of the Union's idiosyncratic constitutional order. It assesses the broad powers with which national courts are endowed, and highlights the dynamic nature of the Union's approach to accommodating potential deviations. Finally, it focuses on an area at the core of national sovereignty, namely defence, and outlines the gradually shifting approach to its economic aspects which the Union institutions have developed over the years.