At small scales, magnetic resonance microscopy investigations have an important role in understanding how the diffusion MR signal is related to tissue microstructure and in developing realistic models. Since its introduction in the mid-1980s, diffusion-weighted magnetic resonance imaging has become one of the main tools of modern neuroimaging studies. By providing data at microscopic scales, diffusion MR microscopy has the potential uncover the biophysical mechanisms underlying neuronal activation. Unlike the diffusion signal decay in a homogeneous medium, the diffusion signal behavior in tissues is not monoexponential. Instead, biexponential functions where empirically found to provide good fits. Studies performed on other large cells also confirmed that the presence of the extracellular space is not necessary to observe the multi-exponential diffusion behavior. Tissue geometrical parameters can be obtained by comparing numerical simulations of simple tissue models with experimental data. Numerous biophysical models have been proposed assuming that the tissue is composed of an extracellular space and spherical, ellipsoidal or cylindrical cellular components.