The EU has been making strong inroads into the realm of security over the last few years. This is a remarkable development, since security matters used to be the preserve of states. The articles presented in this special issue all testify to the breadth of the EU security agenda, as they all try to capture some aspects of the EU’s fast-changing security policies following the entry into force of the Lisbon Treaty on 1 December 2009. In parallel with a broadening of the EU’s security agenda, an increase in supranational security governance in the EU can also be observed. The transition to supranational governance is reached in two ways. First, cross-border security threats generate demand for EU laws, which supranational organisations then supply. Reasons for changes in the EU polity are exogenous shocks, the fact that rule innovations are endogenous to politics, the diffusion of organisational behaviour and models of action, and policy entrepreneurship, whereby institutional entrepreneurs construct and revise ‘policy frames’, which engage other actors and define new relationships between them and chart courses of action. As the articles in this special issue demonstrate, 11 September 2001 provided such a major exogenous shock required for a change in the EU polity, which EU institutions exploited by providing increasing EU legislation, and even, as a by-product, stabilising a European legal order.