While EU legislation is adopted at EU level, its implementation is usually the domain of the member states. However, the EU has opted for coordinating across member states how to implement some parts of this legislation. The Agency for the Cooperation of Energy Regulators (ACER) is an EU agency set up to facilitate cooperation across national energy agencies on the implementation of specific parts of EU energy legi-slation. This report presents the background for the EU’s cooperation in general and on implementation in particular. It then provides rundown of how EU energy policy developed over time and why, before zooming in on the development of ACER. The report offers an overview of ACER’s powers and tasks, and a detailed discussion of the ACER Regulation. The Norwegian discussions surrounding ACER and implementation of the third energy package is also covered. The report con-cludes with a discussion of prospects for earlier and better Norwegian engagement with regard to EU legislation.