This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book addresses some of the more important aspects of the energy performance of commercial and institutional buildings. It can be neither exhaustive nor definitive, but rather provides an in-depth picture of the state-of-the-art in building energy performance. The book examines the available models that relate to the energy performance of buildings, as background to the thinking behind the conceptual framework. Generalized energy models have been proposed which attempt to cross the boundaries embodied in the above classification and take into account the other factors required for a complete modeling of building energy performance. Architectural models have been concerned more with the environmental performance of buildings than with the energy consequences of achieving that performance.