This paper approaches the study of national adaptation to the European Union as a process involving institutional constraints and actors' interactions across levels of decision-making. The argument is that domestic adaptation to the EU is a matter of the ability and willingness to conduct integrative political bargaining rather than a matter of matching institutional structures. The paper provides an empirical case study of the Norwegian adaptation to EU energy sector legislation, the Internal Energy Market (IEM). The various outcomes of different directives in this sector indicate that the structural feature of a particular state or policy sector is inadequate to explain fully variations in national and domestic adaptation to EU legislation. The paper focuses on characteristics of the process of adaptation itself, such as affectedness, policy similarities, bargaining opportunities, and legal proceedings.