The authors present a noninvasive method for tracking neuronal fiber pathways in the living human brain using diffusion tensor imaging (DTI). They developed a technique that reconstructs fiber trajectories by tracking the direction of fastest diffusion from a grid of seed points, selecting tracks that join anatomically or functionally defined regions. The method was applied to various white matter classes, including the corpus callosum, geniculo-calcarine, and subcortical association pathways. The results showed that the tracks covered long distances, navigated through divergences and tight curves, and revealed topological separations consistent with animal studies and retinotopy studies in humans. The approach enhances the power of modern imaging by enabling the study of fiber connections among anatomically and functionally defined brain regions in individual human subjects, providing a more integrated understanding of the organization of the human nervous system.The authors present a noninvasive method for tracking neuronal fiber pathways in the living human brain using diffusion tensor imaging (DTI). They developed a technique that reconstructs fiber trajectories by tracking the direction of fastest diffusion from a grid of seed points, selecting tracks that join anatomically or functionally defined regions. The method was applied to various white matter classes, including the corpus callosum, geniculo-calcarine, and subcortical association pathways. The results showed that the tracks covered long distances, navigated through divergences and tight curves, and revealed topological separations consistent with animal studies and retinotopy studies in humans. The approach enhances the power of modern imaging by enabling the study of fiber connections among anatomically and functionally defined brain regions in individual human subjects, providing a more integrated understanding of the organization of the human nervous system.