The rapid pace of discovery in membrane molecular biology is revealing unexpected complexity at the boundary of the cell. The membrane appears to be more than a thin film separating aqueous compartments or an anchoring site for proteins. Indeed, it shows many indications of being a dynamic, regulatory structure composed of transient lipid ensembles known as rafts or microdomains. These appear to regulate the membrane-protein kinetics which is the basis of many cellular functions. This book describes those regulatory features in detail and presents evidence that they operate in the neuron.