The historical evolution of the energy relationship between Russia and Europe, from Cold War up to the break after 2022, is examined in this thesis. Furthermore, the way this shift reshaped the EU’s security of supply, market design and decarbonisation pathway is analysed. Βy using official statistics, policy documents and academic literature, the thesis maps how interdependency grew around long-term pipeline contracts. It also identifies the episodes that exposed structural vulnerabilities and the broader framework of diversification and energy transition that was triggered by the 2022 war in Ukraine, resulting in the REPowerEU package. It is shown how energy price shocks, storage dynamics and reliance on imports led to structural responses in the EU framework. Specifically, internal market rules, reverse-flow systems, interconnections and storage obligations are included in these responses. They were complemented by the Green Deal that directs investment to renewables, grids and flexibility. Moreover, transatlantic dynamics are taken into account. It is shown how flexible US LNG, alongside with Europe’s regasification capacity and market integration, have provided short-term flexibility. Thus, Russian pipeline volumes could be gradually replaced and storage to be refilled. However, it is expected that gas demand will be in decline because of the increase in electrification, efficiency and environmental protection. Finally, the potential risks and benefits of a limited, conditional re-engagement with Russian energy supply are evaluated. It is seems that Europe’s path forward relies on further electrification by domestic renewables, stronger grids and cross-border connections. The importance of demand-side flexibility is also highlighted, with gas expected to be in a declining, backup role in the future.