The constraints of the European legal and institutional structure for decision-making in energy greatly determine the quality and the dynamics of regulatory practice, in particular the legal tools we use to build markets and how we use them. In this light, this paper aims to analyze two major developments in energy market regulation at the Union level: the creation of a new Agency for the Cooperation of Energy Regulators (ACER) and the intensified use of the EU antitrust rules by the European Commission. It first shows that ACER will not enjoy any real decision-making or veto powers and focuses on existing sources of concern. It then argues that the Commission is increasingly taking a quasi-ex ante regulatory approach through antitrust to fix the shortcomings of sector-specific regulation, even if this might be neither a suitable nor a legitimate approach. The paper concludes on the need to better articulate EU regulatory and competition-based controls to push forward the completion of the single market in energy.