The unique ability of MR to demonstrate both normal and abnormal white matter maturation with a high degree of sensitivity makes it an indispensable tool with which to evaluate children with clinical developmental delay or suspected dysmyelinating processes. Because the noninvasive nature of MRI allows sequential studies to be performed without additional risk to the patient, it offers an unprecedented opportunity to study the process of myelination in normal infants, developmentally delayed infants and to better understand the dysmyelinating disorders, a relatively rare, poorly understood group of diseases.