In an era of big data and smart cities, this book is an innovative and creative contribution to our understanding of urban energy use. Societies have basic data needs to develop an understanding of energy flows for planning energy sustainability. However, this data is often either not utilized or not available. Using California as an example, the book provides a roadmap for using data to reduce urban greenhouse gas emissions by targeting programs and initiatives that will successfully and parsimoniously improve building performance while taking into account issues of energy affordability. This first of its kind methodology maps high-detail building energy use to understand patterns of consumption across buildings, neighborhoods, and socioeconomic divisions in megacities. The book then details the steps required to replicate this methodology elsewhere, and shows the importance of openly-accessible building energy data for transitioning cities to meet the climate planning goals of the twenty-first century. It also explains why actual data, not modeled or sampled, is critical for accurate analysis and insights. Finally, it acknowledges the complex institutional context for this work and some of the obstacles – utility reluctance, public agency oversight, funding and path dependencies. This book will be of great value to scholars across the environmental sectors, but especially to those studying sustainable urban energy as well as practitioners and policy makers in these areas.