The study by Lebel and Beaulieu examines longitudinal white matter maturation in 103 healthy subjects aged 5–32 years using diffusion tensor tractography. Fractional anisotropy (FA) and mean diffusivity (MD) were assessed in 10 major white matter tracts. All tracts showed significant nonlinear development trajectories for FA and MD. Significant within-subject changes occurred in most children and early adolescents, with these changes mostly complete by late adolescence for projection and commissural tracts. However, association tracts demonstrated postadolescent maturation of both FA and MD. Diffusion parameter changes were primarily due to decreasing perpendicular diffusivity, although increasing parallel diffusivity contributed to prolonged FA increases in association tracts. Volume increased significantly with age for most tracts, and longitudinal measures also showed postadolescent volume increases in several association tracts. The observed diffusion parameter changes likely reflect microstructural maturation rather than gross anatomy. The study found that white matter volume increased significantly across the age range, while gray matter volume decreased, leading to no overall significant change in total brain volume. FA and MD showed nonlinear age-related changes, with most tracts demonstrating prolonged development in association fibers. Commissural and projection fibers matured earlier, while association fibers continued to develop into young adulthood. Sex differences were observed, with females having higher FA in the splenium of the corpus callosum and males having higher FA in the cingulum and corticospinal tracts. Tract volume changes were significant for most tracts, with the exception of the superior fronto-occipital fasciculus. Age remained a significant predictor of FA and MD even after including tract volume as a covariate. The study highlights the continued maturation of white matter fibers beyond adolescence, with diffusion parameter changes primarily driven by perpendicular diffusivity, suggesting increased myelination and axonal packing. The findings emphasize the importance of understanding healthy brain development for future studies of abnormal development and its relationship to psychiatric disorders.The study by Lebel and Beaulieu examines longitudinal white matter maturation in 103 healthy subjects aged 5–32 years using diffusion tensor tractography. Fractional anisotropy (FA) and mean diffusivity (MD) were assessed in 10 major white matter tracts. All tracts showed significant nonlinear development trajectories for FA and MD. Significant within-subject changes occurred in most children and early adolescents, with these changes mostly complete by late adolescence for projection and commissural tracts. However, association tracts demonstrated postadolescent maturation of both FA and MD. Diffusion parameter changes were primarily due to decreasing perpendicular diffusivity, although increasing parallel diffusivity contributed to prolonged FA increases in association tracts. Volume increased significantly with age for most tracts, and longitudinal measures also showed postadolescent volume increases in several association tracts. The observed diffusion parameter changes likely reflect microstructural maturation rather than gross anatomy. The study found that white matter volume increased significantly across the age range, while gray matter volume decreased, leading to no overall significant change in total brain volume. FA and MD showed nonlinear age-related changes, with most tracts demonstrating prolonged development in association fibers. Commissural and projection fibers matured earlier, while association fibers continued to develop into young adulthood. Sex differences were observed, with females having higher FA in the splenium of the corpus callosum and males having higher FA in the cingulum and corticospinal tracts. Tract volume changes were significant for most tracts, with the exception of the superior fronto-occipital fasciculus. Age remained a significant predictor of FA and MD even after including tract volume as a covariate. The study highlights the continued maturation of white matter fibers beyond adolescence, with diffusion parameter changes primarily driven by perpendicular diffusivity, suggesting increased myelination and axonal packing. The findings emphasize the importance of understanding healthy brain development for future studies of abnormal development and its relationship to psychiatric disorders.