Detergents are used in molecular biology laboratories every day. They are present in cell lysis buffers (e.g., in kits for plasmid isolation), in electrophoresis and blotting buffers, and, most importantly, they are used for cleaning laboratory glassware and the hands of the laboratory staff. For these routine applications, a detailed knowledge of detergent properties is not necessary-they just work. When it comes to the isolation and purification of membrane proteins, one cannot rely on routine protocols. Many membrane proteins are only stable in a small number of different detergent buffer systems, and worst of all, different membrane proteins prefer very different detergents. Unfortunately, detergent properties are considered the domain of colloid science or physical chemistry, and thus, while the available amount of physico-chemical data on detergents is astounding, this data is rarely compiled in a way that is useful to biochemists. The aim of this chapter is to provide an overview of the physical and chemical properties of detergents commonly used in membrane protein science and to explain how these properties can be exploited for protein purification.