The Agency for the Cooperation of Energy Regulators (ACER) was launched in 2011 to contribute to the establishment and functioning of the eu’s internal energy market. This study aims to evaluate ACER’s success in establishing and ensuring the functioning of the internal energy market, both from ACER’s and its stakeholders’ (energy regulators and energy traders in the eu member states) perspective. The paper includes the results of interviews with nine national regulatory authorities (NRAS) and ten energy traders fromthe eu member states. This research revealed that, based on their perception, ACER is actually crucial for the operation of the internal market. In addition, NRAS and energy traders mostly agree that the eu has a nearly functional internal energy market and that ACER is vitally important for the functioning of the internal market.