Book excerpt: Cell surface membranes have long been characterized as two-dimensional fluids whose mobile components are randomized by diffusion in the plane of the membrane bilayer. Recent research has indicated that cell surface membranes are highly organized and ordered and that important functional units of membranes appear as arrays of interacting molecules rather than as single, freely diffusing molecules. Mobility and Proximity in Biological Membranes provides an overview of the results obtained from biophysical methods for probing the organization of cell surface membranes. These results are presented in the context of detailed treatments of the theory and the technical demands of each of the methods. The book describes a versatile and easily applied mode for investigating molecular proximities in plasma membranes in a flow cytometer. Its analysis of lipid fluidity and viscosity of membranes and the rotational mobility of proteins offers intimate insight into the physical chemistry of biological membranes. The electrophysiology of lymphocytes is presented with focus on its importance in different diseases. New techniques are described, and new data, new possibilities, and future trends are presented by world experts. This book's chapters can serve both as guides to the existing literature and as starting points for new experiments and approaches associated with problems in membrane function.