Recent EU integration in energy market policy, combined with significant heterogeneity across national energy sectors, begs the question of the extent to which and why there is differentiated integration of EU energy market policy. The chapter by Torbjørg Jevnaker assesses internal and external differentiated integration across EU energy market laws adopted during the years 2009– 2019 and examines whether member states have opted out of, and third countries have been induced to participate in EU rules in this policy area. Differentiated integration in the EU energy market policy is found where there is low interdependence and few externalities as well as in undeveloped energy systems. Differentiation is thus driven by demands from a relatively small yet coherent group of member states, which, due to low incidence of externalities, can be supplied by the group of member states that choose to proceed with integration. The same pattern is found in select third countries, although sovereignty- sensitive concerns play a bigger role in accounting for whether they participate in (parts of) an EU energy market policy. Differentiated integration is low due to the high level of interdependence across countries, as well as due to a consensus orientation in negotiations on secondary EU legislation. The latter ensures that widespread resistance is accommodated via flexibilities (and by keeping overall integration at a lower level) rather than opt- outs. Thus, not all heterogeneity translates into differentiated integration.