The book summarizes the results of an international research project - the first Europe-wide Delphi study on future developments in the energy sector (EurEnDel). With a time horizon of 2030, this expert survey not only provides a useful perspective on long-term developments of energy technologies, but also evaluates these technologies against different sets of social values or 'visions'. Close to 700 energy experts from 48 countries participated in this two-round, web-based Delphi exercise. Thus the results represent a broad European perspective on the challenges that Europes energy system will be facing over the next two and a half decades. In the survey and in the analysis of the results, equal emphasis was placed on the technology-push perspective 'What will the future be like?' and the normative social-pull perspective 'What should the future be like?'. In this respect, considerable attention was given to the different responses received from technology experts, generalists and decision makers. The results of the Delphi study were interpreted on the background of three qualitative scenarios of Europes energy futures up to the year 2030. In a world of uncertainties, EurEnDel recognized the impracticality of referring to a single energy scenario. For this reason it adapted the classical Delphi approach, employing a variety of foresight approaches both in the design phase and in the later analysis of the results The survey covers 20 emerging technologies from all parts of the energy system ranging from energy supply, grid and storage issues, and energy demand for transportation, housing and industry. The covered key technologies were assessed in terms of time horizons of development, their potential impact and the means which are needed to support their development.