This book stems from a research project funded by the Spanish Ministry of Science and Innovation that aimed to study energy networks in Europe (gas and electricity). The project was motivated by Directives 2009/72 and 73 of the European Parliament and of the Council, of 13 July 2009, concerning common rules for the internal market in electricity and natural gas (the so-called «third energy package»). The effective creation of an internal energy market (which should be completed in 2014) is inseparable from the process of constructing and using electricity and gas networks at European scale. And this effort calls for the «complicity» of the Member States. Hence the close relationship between European Union law and the relevant National laws, the narrative thread of this book, which aims to address the following topics: introduction or historical context; the division of competences; regulatory agencies; the construction of the networks: planning of the networks and coordination with territorial planning; the unbundling of activities; the right of access to the networks; cross-border transmission. The end result is a «frieze» that highlights the characteristic features and evolution of the National laws, held up against the demands of European Union law. We hope that this «explanation» might contribute to a simpler and quicker implementation of the internal market and, particularly, the construction and use of the major gas and electricity networks in the European Union. The authors are professors of Administrative law at Universities in Spain, Italy, France and Germany, all recognised specialists regarding these subjects.