Myelin found in the gray matter of the cortex is called intracortical myelin. Overall brain myelination begins in utero and continues into the fifth decade of life in humans. Intracortical myelination follows a specific pattern, with primary sensory and motor areas myelinating well before association areas. It plays an important role in modulating behavior and cognitive functioning and has a critical period of development. In healthy humans the development of intracortical myelination follows an inverted-U trajectory, peaking within the mid-30s. In bipolar disorder, this trajectory is significantly blunted across many cortex regions, indicating an age-related disruption in the global pattern of development. These deficits have further been associated with worse verbal memory in bipolar disorder, which is one of the most impaired cognitive domains in this population. Myelin in the brain is sensitive to many exogenous and endogenous factors. For instance, antipsychotics have been shown to modulate myelin in schizophrenia, and there are preliminary findings that the same might be true in bipolar disorder. Prospective analyses will help determine the directionality and therapeutic potential of the association between intracortical myelin, behavioral outcomes, and modulating mechanisms.