Bringing together leading experts from across the UK and Europe, this book provides a comprehensive analysis of the impact of Brexit on the energy sector in the UK and in the European Union and its Member States. In recent decades, the trend within the EU has been towards greater integration and liberalisation of energy markets. Through the development of the Union's internal energy market and the funding of cross-border energy infrastructure, EU membership facilitates cross-border trade in energy, promotes security of energy supply, and, via the European Atomic Energy Community (Euratom), allows EU Member States to trade in nuclear material for energy production. Brexit changes all of this. The significant level of integration and interdependence in EU energy policy means that the UK's departure from the Union poses many challenges for the UK, the EU, and EU Member States. While certain energy-related arrangements have been addressed, the relationship between the UK and EU in the energy sector has been changed fundamentally. In this context, important and interrelated questions arise, such as the following: Under what terms will energy trading between the EU and UK now take place? What access will the UK have to EU energy markets? What does Brexit mean for the security of energy supply? What does the UK's departure from Euratom mean for nuclear research? Can the crossborder single energy market (SEM) on the island of Ireland continue following Brexit? What implications does Brexit have for renewables, the environment, and climate change? Brexit comes at a time when the European energy sector is undergoing the processes of decarbonisation and energy transition. This book offers researchers, legal practitioners, and policy advisers in-depth understanding of the interplay between these challenges and Brexit.