This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book discusses factors that affect the link between energy and gross domestic product growth, from the perspective of a multi-output production function and explains how energy intensity declined while energy demand increased over time. It focuses on energy efficiency, economic growth, and economy-wide rebound effect. The book reviews the climate policy literature, explains why models generate diverse results with varying cost estimates, and presents main determinants of climate policy costs. It shows that economic, technological, environmental, and societal assessments have different spatial scales but are very interdependent in energy systems modeling, creating challenges for policy makers and researchers. The book also explains how renewable energy policy evolved and also takes up the efficacy of various renewable policies.