ACER (Agency for the Cooperation of Energy Regulators) was established as an EU agency. It is the successor of the former advisory group to the European Commission (ERGEG). All of the EU regulatory authorities in the field of electricity and gas are compulsory members of ACER. 

ACER plays a key role in developing EU-wide network and market rules in order to foster competition. The agency coordinates regional and cross-regional initiatives to support the integration of the European market. It also monitors the work of the European Networks of Transmission System Operators (ENTSO), particularly by issuing opinions on their EU-wide network development plans. Lastly, ACER monitors the well-functioning and transparency of the EU Internal Energy Market in general and oversees the wholesale energy market in particular. 

ACER can issue non-binding opinions and recommendations to NRAs, transmission system operators (TSOs) and the EU institutions. It can also make binding individual decisions about cross-border infrastructural issues. Upon the request of the European Commission, it can adopt draft framework guidelines, which form the basis of network codes. 

The seat of ACER is in Ljubljana, Slovenia. ACER’s decisions and other documents can be found on its website.