This chapter introduces the principles of absorption, including a basic explanation of the physics and some fundamental formulations that will be used in later chapters. Since this book is aimed at both practitioners and researchers, most chapters begin with an application-driven, qualitative description, followed by a quantitative description of the technology and design. Following this type of philosophy, this introduction to absorbers is written from an application or case study perspective. The style is intended to make the more theoretical sections more palatable and avoid starting with a chapter labelled A little light mathematics. The chapter will also introduce some of the issues concerning the design, prediction, and measurement of absorbers that will be treated in more detail in future chapters.