Myelin is a key evolutionary acquisition that underlay the development of the large, complex nervous systems of all hinged-jaw vertebrates. By promoting rapid, efficient nerve conduction, myelination also made possible the development of the large body size of these vertebrates. In addition to increasing the speed of nerve conduction, myelination has emerged as a source of plasticity in neural circuits that is crucial for proper timing and function. Here, we briefly describe the organization of myelin and of myelinated axons, as well as the functions of myelin in nerve conduction and neural circuits, and consider its potential evolutionary origins.