The Agency for the Cooperation of Energy Regulators (ACER) has been established in 2009 in the Third Liberalization Package of Energy Sector. The Agency became fully operational in 2011. After 5 years of its functioning the Author presents his opinion on general issues like legitimacy of the European Union regarding the tendency of creating new European agencies (among them ACER) and the problem of the common belief in rule of experts in the typical field of politics. Subsequently the paper handles the history of setting up the Agency and the description of its bodies and competencies in order to present the achievements of ACER within 5 years of its work. The Author concludes with positive statement: ACER meets its goals, encompassing the principle of subsidiarity and helping the Commission and National Regulatory Authorities in realizing the aim of establishing the common energy market in European Union