Until the advent of MR imaging, knowledge of the structure of myelin and the process of myelination were of little importance to the neuroradiologist. Other than some mild changes in the attenuation of white matter, myelination resulted in no significant alterations of CT (1) or sonographic studies. MR studies, on the other hand, have been increasingly used for pediatric brain imaging. MR imaging's greater sensitivity to small changes in the water content of brain tissue, to changes in the binding of free water (revealed by magnetization transfer), and to the extent and anisotropy of water diffusion (revealed by diffusion imaging) has cast new light on this very complex and important molecule. Assessing myelination has become a key component of evaluating the child with delayed development. Moreover, better understanding of the nature of myelin and the effect of its different components on MR imaging parameters may help us to understand and diagnose inborn errors of metabolism better. In this review, I discuss what is known regarding the function and structure of CNS myelin and the effects of the various components of myelin on the signal imparted to the MR image. Summary Abnormalities of myelin can cause a wide variety of disorders of the nervous system. MR imaging is a powerful tool for the study of myelin and its disorders. However, only by understanding the physiologic properties and structure of myelin can we use MR imaging to its fullest capacity for studying patients with myelin disorders. In this review, I have discussed the structure of myelin as it relates to MR imaging of normal myelination and to neurologic disorders resulting from abnormalities of myelin. Thinking of myelin and its disorders in this manner will be critical to using MR imaging techniques optimally to diagnose and study these disorders further.