Before we can begin discussing energy sources and their future, we must first gain a clear understanding of exactly what energy is—and what it is not. This chapter provides an introduction to the concepts of energy: what it is, how different aspects of it were discovered over centuries of time, and what the different forms and sources of energy are. It discusses the various ways that energy can be transported and how it can be converted from one form to another. It explains how different types of energy are typically measured and compared and discusses the many ways energy is utilized in society and how the trends of use have changed over time, and introduces what the implications of those changes mean to society today. Finally, it offers insight into the conservation of energy, why energy efficiency has become such an important issue today, and why switching to renewable energy after relying for so long on traditional resources makes so much sense in the rapidly-changing world that we live in.