Energy is of vital importance to the economy of the European Union (EU), safeguarding stability and prosperity for its Member States and citizens. Cooperation in energy policy was enshrined as a cornerstone in the EU's founding treaties, through the European Coal and Steel Community (ECSC) and the Euratom Treaty. However, energy policy was initially characterised by limited and shallow integration, with the first measures to integrate the policy area implemented 40 years after the foundation of the ECSC. Within this context, the Commission has taken an ambitious stance and proposed an umbrella institution to embrace the various dimensions of energy policy: the European Energy Union. This thesis aims to assess the factors and mechanisms that lead to integration, including both the internal and external dimension of European energy policy. In order to achieve this, it utilises Historical Institutionalism as a theoretical tool, which stands between rational choice and sociological approaches to the study of institutions. Based on the notions of path dependence, institutional lock-in and institutional inertia, the project analyses how the institutional matrix both constrains and enables different actors’ political strategies. This is accomplished through a comprehensive analysis of primary and secondary legislation, historical documentation, communications, supplemented by primary data gathered from interviewing decision-makers and experts in Brussels. Process-tracing is applied to test four hypotheses regarding integration of the energy sector from the inception of the ECSC to the Energy Union. It assesses under what premises initial legislative proposals were made, identifies critical junctures that enabled institutional change, and determines which key players were pivotal in formulating policy proposals, ultimately culminating in the policy package for the Energy Union. Ultimately, this PhD thesis seeks to answer the question under what conditions does integration occur in the policy area of energy over time?