Illustration of the distinct phenomena of short-time diffusivity, restriction, anisotropy, exchange, and flow that all occur in living biological tissues and may be independently and unambiguously interrogated with appropriately designed advanced diffusion encoding methods (see Chapters 2–7). The short-time diffusivity is determined by the local chemical composition of the aqueous phase and biomembranes (grey lines) influence the evolution (bottom to top) of the diffusion propagators (“ink spots”) on the millisecond time scales and micrometer length scales relevant for diffusion MRI. The sizes, shapes, and orientations of the spots are intuitively related to the underlying tissue properties. (a) Free diffusion in pure water with diffusivity 3.0·10−9 m2 s−1 at 37 °C giving isotropic root-mean-square displacements of 0.77, 7.7, and 17 μm during the times 0.1, 10, and 50 ms. (b) Isotropic tissue with densely packed spherical cells restricting the size of the spot while preserving its spherical shape. (c) Anisotropic tissue with orientationally ordered elongated cells limiting water motion mainly in the horizontal direction. (d) Elongated cells limiting water motion in the vertical direction. (e) Finite membrane permeability allowing the molecules to exchange between the intra- and extracellular spaces. (f) Laminar flow with parabolic velocity profile superposed on the diffusion.