Ls book is an account of what physical chemistry h . . to say about the structural, electrical and transport properties of biological membranes and their simplest model-the lipid bilayer. The accent throughout is on basic ideas. In contrast to the essentially descriptive ap- proach characteristic of texts on membrane biochemistry, our underlying themes are the role of force and entropy in maintaining membrane organization, in determining the electric fields and ionic environment of membranes, and in regulating the passage of molecules and ions across membranes. Although experimental findings will always be the touch- stone against which theory will be tried, no attempt is made to present an exhaustive survey of experimental data. On the other hand, there is discussion of the nature and limitations of the results obtainable by the major laboratory techniques. The treatment is at the level of an advanced undergraduate course or an introductory survey suitable for post- graduate students carrying out research in biochemistry, biophysics, or physiology. The mathematical demands on the reader are trivial. The few forbidding equations appearing in Chapter 7 are soon whittled away to simple practical expressions. Although the current-voltage characteristics of nerves are traditionally the province of biophysics rather than physical chemistry, certain aspects relevant to the electrical activity of nerves are nevertheless included in this text, namely, mem- brane and diffusion potentials and conductivity fluctuations. Where rival theories exist, conflicting convictions have been presented, but not necessarily accorded equal approbation. The author has a viewpoint.