This book represents accumulutation of experience, observations, research, and knowledge collected over 25 years. It is based on countless energy audits, studies in both real buildings and laboratory, cooperation with energy efficiency and climate policy makers, owners of buildings and project funders. The book explores all above mentioned factors that are part of a very complex system in which many actors have to make difficult decisions. The book tries to answer questions raised not only by the researchers, but also building professionals about material and technology selection, building owners and managers on energy efficiency project types, and energy efficiency policy tools for policy makers to reach the goals. The structure of this book starts with the description of the types of buildings and their energy consumption, followed by an overview on microclimate in buildings, building physics, evaluation of energy efficiency measures, energy efficiency of building envelope and engineering systems and finally giving an example of energy service contract. The book is intended for all those engaged in building energy efficiency issues, including researchers, building owners and managers, producers of materials and Technologies, construction industry professionals, and policy makers.