This chapter focuses on energy—perhaps the most important idea in all of science. It is used by every scientific discipline and applies to all known natural phenomena. Students hear and use the word “energy” from a very early age, but developing an understanding of energy as a scientific concept requires careful instruction over the course of many years. If students develop a deep understanding of a small set of fundamental energy ideas over time by exploring real-world phenomena, they are well positioned to continue using the concept as they reason about energy-related decisions in their everyday lives and participate in energy policy discussions that affect our environment. A list of materials, teacher background information, assessment, standards, safety, and references are provided. This book selection also includes the Table of Contents, Foreword, Acknowledgments, About the Editors, Contributors, Appendix, Image credits, and Index.