The European Union Agency for the Cooperation of Energy Regulators (ACER) is an Agency of the European Union created under the terms of the Third Energy Package of 2009.

[2] The basis for the establishment of ACER is Regulation 713/2009 of the European Parliament and the Council of 13 July 2009.

The regulation describes the establishment and the legal status, tasks, organisation, and financial provisions.

[3] It was reestablished in 2019 by directive (EU) 2019/942[4] ACER is the second intergovernmental organization headquartered in Slovenia.

The first was the International Center for Promotion of Enterprises (ICPE).