Despite common misconceptions, MRI is a spectroscopic tool, able to resolve water populations according to their chemical and physical interactions with the environment. These interactions can be used to indirectly infer the local composition and microstructure of heterogenous, opaque, and sensitive materials, such as the living human brain. Because a typical MRI voxel is on the order of millimeters, it contains several distinct water microenvironments. Conventional MRI methods only provide voxel-averaged and macroscopic information; these methods cannot disentangle intra-voxel heterogeneity on the basis of water mobility and local chemical interactions. This chapter details the integration of multiple MR dimensions; this approach enables microscopic variability within biological materials to be obtained and preserved. The use of a nonparametric, phenomenological representation of the multidimensional MR signal makes no assumptions about tissue structure, therefore allowing a direct characterization of complex heterogenous biological systems.