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...