Diffusion tensor magnetic resonance imaging is an imaging modality that measures the diffusion properties of water molecules in tissues noninvasively. Water diffusion is affected by tissue constituents, such as macromolecules, membranes, organelles, as well as by tissue microstructure, architecture, and organization. From the quantities measured with diffusion tensor-magnetic resonance imaging, one can infer information about brain tissue that cannot be obtained using conventional, compositional-based (eg, proton density and chemical spectroscopy), or relaxometry-based (eg, magnetization transfer, T1, T2, and T2*) magnetic resonance imaging methods. Understanding the relationship between a measured water diffusion pattern and the underlying histological features of the tissue, however, is not simple. In the absence of a robust and comprehensive model of water diffusivity in biological tissues, the biological interpretation of diffusion measurements relies on empirical evidence. Herein, the properties of several diffusion tensor-derived quantities are reviewed, together with the experimental evidence that helps clarify their relationship with the underlying properties of brain tissue.