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Phospholipid molecules in the anhydrous state and swollen in water have a critical transition temperature above which they are in a fluid condition and below which the chains crystallize. This transition temperature is related to the chain length and degree of unsaturation. These experimental observations led to the concept of Biomembrane Fluidity being put forward in 1966 (Proceedings of the Royal Society A 290, 115-142). In this way it was possible to rationalize the observations that the lipid chains in some membranes are highly unsaturated, whereas in other membranes they are longer and more saturated, thus giving rise to membranes of different fluidity. This led directly to methods to measure fluidity of membranes by n.m.r. and e.s.r. spectroscopy as well as fluorescent probes. The concept of biomembrane fluidity therefore emphasized the dynamic nature of membranes, the fluid nature of the chains as well as the potential for rapid lipid diffusion. These ideas were later incorporated into the popular model for biomembrane structures, i.e. the Fluid Mosaic model put forward by Singer and Nicholson. This volume, edited by Kates and Manson, includes twenty-three chapters on various aspects of membrane fluidity and its implication in membrane function. The book is divided into three main sections. The first is on Physical Studies of Membrane Fluidity, the second, Regulation of Membrane Fluidity and the third, Correlation of Membrane Activity and Physiological Activity. In the first section of the book, various physical techniques are described to study fluidity in model and biological membranes. These include the techniques of calorimetry, n.m.r. spectroscopy, fluorescence polarization and e.s.r. spectroscopy. These are contributed to by J.