This core chapter accounts for most of the newest technology related to the measurement of membrane permeability of molecules. The chapter considers in depth the PAMPA (parallel artificial membrane permeability assay) approach, the high-throughput method recently introduced by Manfred Kansy et al. of Hoffmann-La Roche. Chapter 7 is the first thorough account of the topic and takes up almost half of the book. The ‘double sink’ method is described. Original equations are derived, including those covering gradient-pH methods. Nearly 4000 original measurements are tabulated in the chapter.