Enforcement of European Union (EU) energy law is a two-sided affair. In the big picture, the EU has successfully liberalized and integrated the European energy markets. It has recognized new rights for energy consumers and ensured more transparent and liquid energy trading. Whereas the energy sector used to be a national domain, today energy is traded and transported across borders, based on common EU rules. Energy is a strategic domain for States individually and for their collaboration with the EU, as revealed by the energy price and supply crisis that started in the winter of 2021/22. Yet beyond areas of common interest, differences between the EU and the Member States concerning energy policy goals remain. As per its usual approach, the EU endeavours to create an internal energy market (IEM) with minimum rights for market players and consumers, and to overcome nationalistic reservations against its energy policy goals by harmonizing the legal framework. This has resulted in a comprehensive and prescriptive legal framework that relies on a mix of substantive and technical legal requirements to facilitate enforcement. Nevertheless, the implementation of the legal framework, as well as the investigation and sanctioning of breaches, starts at Member State level. In this chapter, we explain the progressive and accelerating evolution of EU energy law from non-existence to an established vein of regulation in a matter of a few decades. We explore factors for the success in enforcing EU energy law, notably with liberalizing the sector and integrating the segregated national energy markets. Finally, we comment on current trends and outline possible legal limits to today’s enforcement strategy in EU energy law. In our view, the EU is overall successful regarding enforcement in the energy sector. However, our examples illustrate that advancement in several critical areas is delayed and erratic. The simple fact that the Commission’s and ACER’s resources are limited speaks against a hard enforcement strategy to address remaining enforcement gaps at EU level. Such an approach could also entail a hardening of opposing positions. Instead, the EU should build on the consensus represented in the adopted harmonized legislation to overcome remaining points of contention. This would likely contribute more to continued success with energy law enforcement than any particular enforcement tool.