  Diffusion-weighted MRI is the forerunner of the rapidly developed
microstructural MRI aimed at in vivo evaluation of the cellular tissue
architecture. This brief review focuses on the spatiotemporal scales of the
microstructure that are accessible using different diffusion MRI techniques and
the need to weight the measurability against the interpretability of results.
Diffusion phenomena and models are first classified in two-dimensional space
(the q-t-plane) of the measurement with narrow gradient pulses.
Three-dimensional parameter space of the Stejskal--Tanner diffusion weighting
adds more phenomena to this collection. Modern measurement techniques with
larger number of parameters are briefly discussed under the overarching idea of
diffusion weighting matching the geometry of the targeted cell species.
