This paper analyses the design and functioning of the EU Agency for the Cooperation of Energy Regulators’ board of appeal, and provides a record of the key issues of substance and procedure it faced, from the inside perspective of the authors, as former chairman and vice chairman of this body, from November 2016 to October 2021. As in some other EU decentralised agencies, the BoA is an independent panel of specialists appointed to review the appeals against the agency’s decision, and a mandatory gateway to the General Court. It’s specificities stem from those of ACER’s role in the construction of energy markets and of the place of the decisions it takes in the architecture of the EU energy policy : despite being labelled as “individual decisions” many of ACER’s decisions are of general application and have major impacts on energy markets. In the context of the implementation of network codes, the BoA was faced with an increasing number of cases (29 appeals, which were consolidated into 19 decisions) on complex issues. The time limit in which it must issue its decisions has been extended from 2 to 4 months to enable it to progressively adjust the depth of its scrutiny to the appropriate level. Outstanding questions remain on the resources allocated to the board and on the clarification of its powers and procedures.