Key concepts in energy considers the process of energy-substitutions and analyzes it as a process of complementary usages, hybridization and technological mixes. The ex-post view tends to focus on replacement from among alternative energy-technologies and is basically innovation-centric. This means that little attention has been given to factors such as the windows of opportunities created by governments, inventors and entrepreneurs. This book highlights how key energy concepts surfaced, tracing their evolution throughout history. It encompasses four economic concepts (rebound effect, energy intensity, marginal cost pricing and levelized cost accounting) and four technological-engineering concepts (primary/final energy, technological hybridization, last gasp and probable oil reserves). The main benefit from reading the book is a cross disciplinary overview of energy fundamentals in a short and focused reading.