This chapter focuses on the use of diffusion MRI to study individual differences in white matter microstructure in the healthy human brain. White matter pathways play an important role in the human brain by connecting spatially separated areas of the central nervous system and enabling rapid and efficient information exchange. Recent developments in magnetic resonance imaging, such as diffusion imaging, have allowed their structural properties to be probed in vivo. With the help of refined imaging and analysis techniques, it has become apparent that these differences are meaningful and can be related to behavioral and functional differences. Even in the brains of non-expert healthy adults, inter-individual variation in gross brain structure is shown to correlate with normal variation in specific behavioral abilities. The complex non-linear dynamics in such a highly connected system as the brain make it difficult to predict the effect of subtle changes in conduction speed, signal attenuation, and frequency spread on its functioning. The current research supports the hypothesis that individual differences in white matter structure are behaviorally relevant and that they can be studied in vivo with diffusion MRI. A better understanding of the behavioral relevance of white matter structure will not only contribute to a better understanding of the healthy brain but also promises to benefit patients recovering from brain injury or illness. In vivo markers of white matter integrity could be used as measures of recovery and responsiveness to treatment. It might also be possible to predict responsiveness to treatment and thereby allow better decisions to be made regarding the most effective treatment strategy.