European Union (EU) energy security is confronted with major challenges, such as the depletion of hydrocarbon reserves within Europe, growing regional competition for fossil fuel resources, the combat against climate change and the need to guarantee affordable energy prices. The combination of the underlying political, economic and environmental constraints has increased the importance of supplies of the least carbon-emitting fuel, natural gas, from and through the EU’s neighbourhood. While hydrocarbons make up for two-thirds of the EU’s energy mix, more than half of this share comes from neighbouring countries (International Energy Agency 2013). However, Member States’ energy mixes differ substantially, as do their path-dependent import dependencies and relations with external suppliers. Based on this, Member States feature divergent interests in the field of energy, which is why a common external energy policy did not find its way into the EU treaties and is not clearly spelled out (Baumann and Simmerl 2011). At the same time, repeated conflicts between Europe’s main supplier, Russia, and the major transit states, Ukraine and Belarus, have revealed the vulnerability of EU supply security towards politico-economic conflicts in the neighbourhood. Due to the enduring political uncertainties regarding the future of Russia–Ukraine relations, the risk of supply cut-offs remains high. This is particularly true for Central and Eastern European countries, whose energy supply is characterised by a high dependence on Russian gas imports and limited supply alternatives (Kovacevic 2009). In addition, resource nationalism, as well as turmoil and instability, after the Arab uprisings in 2011, have demonstrated the fragility of hydrocarbon supply chains in the southern neighbourhood, in terms of drops in production and attacks on energy infrastructure (EU Institute for Security Studies 2014). Against this background, pursuing an effective EU external energy policy in the neighbourhood has become a major security concern for the EU.