Although most people are aware of the value of developing new energy technologies, the importance of assessing such technologies is only just beginning to be recognized in full. This book, illustrated by real-life examples, fulfils two main objectives. Firstly, it provides an in-depth summary of energy system evaluation methods, the result of decades of work in this area, for the use of researchers, engineers and anybody else interested in the energy sector. Secondly, the vicious cycle of neglect towards in situ evaluation is broken. This neglect is due to its unjust reputation for being "thankless work": longwinded, expensive, difficult to exploit and undervalued. By scientifically organizing experience acquired over more than 30 years, Energy Transition highlights the considerable usefulness of the approach, not only economically, but also from a human standpoint.