We investigate the impact of competition policy enforcement on the functioning of European energy markets while accounting for sectoral regulation. For this purpose, we compile a novel dataset on the European Commission's (EC) and EU member states' competition policy decisions in energy markets and combine it with firm- and sector-level data. We find that EC merger policy has a positive and robust impact on (i) the level of competition, (ii) investment and (iii) productivity. This impact, however, only shows up in low-regulated sectors. Other competition policy tools-EC state aid control and anti-trust, as well as all member state policy variables-do not have a uniform effect on energy markets. Our findings are consistent with the idea that the EC's merger policy actions have been used to overcome obstacles to a well-functioning EU energy sector and may have contributed to the overall development of gas and electricity markets in Europe.