This introductory chapter provides an overview of the strategy of protecting group chemistry. In general, a protecting group is introduced onto a functional group to block its reactivity under the experimental conditions needed to make modifications elsewhere. The conditions under which a particular protecting group is either removed or left unaffected must be known; therefore, this book is organised around the mechanistic principles that influence protecting group reactivity. The choice of protecting group is heavily influenced by the type of reactivity that must be blocked, which requires a knowledge of the characteristic reactivity of each functional group. The chapter then looks at protecting group devices and temporary protecting group. Identification of a device within a given protecting group allows one to predict whether the protecting group will be labile (or stable) under specified reaction conditions.