The division of the mature mammalian brain and spinal cord into regions of "white" matter and "gray" matter is observable upon the most cursory inspection. Microscopic examination indicates that the "white" matter regions are characterized by a multilamellar sheath (myelin) which sur rounds the axons. A morphologically similar myelin imparts the white color to tracts of the peripheral nervous system, although, as is empha sized throughout the book, there are very significant morphological and chemical differences between central and peripheral myelin. The rapid postnatal accumulation of myelin is temporally associated with increasing functional capacity and presumably indicates some important role related to neuronal activity. The ability of myelinated axons to conduct electrical impulses much more rapidly and with much lower use of energy, relative to unmyelinated axons, is now generally accepted. As is evident from the above capsule description, a considerable body of information concerning myelin is available. However, as in other areas of research the unknown expands before us more rapidly than we are able to add to our core of knowledge. This is a corollary of the axiom that a good experiment raises more questions than it answers. Among the areas currendy under investigation in many laboratories are the following: interaction of myelin-forming cells with neurons, assembly and metabo lism of myelin, molecular architecture of myelin, and the details of the functional role of myelin.