Since 1992, Justice and Home Affairs ( JHA) has been the fastest growing policy area within the European Union. As a result of its particular opt-out from this area, Denmark has gradually been excluded from EU cooperation on everything from immigration policy to civil law and to police cooperation. In principle, the JHA opt-out sets Denmark free to pursue a different policy than the rest of the Union with regard to matters such as asylum and immigration. Yet in practice the picture is far from that simple. As JHA cooperation has become more ambitious, the ability of the opt-out to ensure legal immunity is more and more challenged. Furthermore, it has become increasingly clear that the opt-out excludes Denmark from cooperation in areas where shifting Danish governments would otherwise prefer to participate and that it limits Denmark’s room for manoeuvre in other areas of European cooperation. The Treaty of Lisbon provides Denmark with an opportunity to replace the current opt-out with a more flexible opt-in. Yet, exploring the British and Irish experiences with a similar model, it is far from certain that such a change will address all challenges when it comes to balancing autonomy and integration.