In this book Hannah Kruimer analyses the application of the legal principle of non-discrimination in the context of energy network operation. Since the early 1990s the duty not to discriminate has applied to energy network operators, in order to achieve a liberalised European energy market, in which European consumers have a free and real choice of energy supplier. This book provides guidance to those working in the context of the non-discrimination obligation, such as energy network operators, regulatory authorities, national courts and other energy market players, as well as those studying the rules for (academic) research purposes. The book's conclusions serve as a tool for critical consideration and offer suggestions for improvements to the legal framework and its application on a European as well as a national level. It is thus of practical as well as academic relevance. Several questions are answered in this book, including why energy network operators have a non-discrimination obligation in the context of energy market liberalisation, how European law has tried to remove and control the discrimination problem since the early 1990s and when different treatment of energy network users amounts to 'forbidden' discrimination? The book's conclusions are underpinned by comparisons with competition law, public procurement law and telecommunications law, as well as a case-study on how energy network operators and regulators in several Member States currently interpret and apply the non-discrimination obligation.