Providing a coherent and realistic vision of the implications of the energy issue in the future development of our cities; places emphasis on the links between very specific and technical topics and the most challenging issues concerning energy savings and the transition to a low carbon society. A great part of the built environment in most European cities consists of buildings dating from 60s to early 90s, for residential purposes: this stock clearly presents several problems related to its functional layout, as well as its energy/thermal behaviour. Applying sustainable and energy savings principles to retrofitting interventions on the stock above is one of the most urgent challenges to be met in the very near future. Giving some examples and case studies, this book investigates retrofitting interventions in terms of energy balance: from design strategies to choice of materials and components; from market trends analysis to economical assessment, from the targeted energy performance to the energy investments needed for achieving it. The reader will benefit from the real life experiences and related results described in this book and acquire all the tools for a constructive evaluation of the different options available to him/her, when faced with retrofitting interventions, thanks to a global view of all the issues involved.