This chapter is intended to introduce the fundamental principles of absorption, along with a basic explanation of the physics behind the absorption processes and some fundamental formulations which will be used in later chapters. Since the book is aimed at 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 introductory chapter on absorption is written from an application or case study perspective. The style is intended to make the more theoretical sections more palatable. Rather than start with a section labelled ‘A little light mathematics’ – which in most books is anything but light – the mathematical explanations will be formed around application examples. 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.