The CNS is extremely responsive to an ever-changing environment. Studies of neural circuit plasticity focus almost exclusively on functional and structural changes of neuronal synapses. In recent years, however, myelin plasticity has emerged as a potential modulator of neuronal networks. Myelination of previously unmyelinated axons and changes in the structure of myelin on already-myelinated axons (similar to changes in internode number and length or myelin thickness or geometry of the nodal area) can in theory have significant effects on the function of neuronal networks. In this article, the authors review the current evidence for myelin changes occurring in the adult CNS, highlight some potential underlying mechanisms of how neuronal activity may regulate myelin changes, and explore the similarities between neuronal and myelin plasticity. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 93-107, 2018.