Diffusion and perfusion MR imaging are now being used increasingly in neuro-vascular clinical applications. While diffusion weighted magnetic resonance imaging exploits the translational mobility of water molecules to obtain information on the microscopic behaviour of the tissues (presence of macromolecules, presence and permeability of membranes, equilibrium intracellular–extracellular water,...), perfusion weighted imaging makes use of endogenous and exogenous tracers for monitoring their hemodynamic status. The combination of both techniques is extremely promising for the early detection and assessment of stroke, for tumor characterisation and for the evaluation of neurodegenerative diseases. This article provides a brief review of the basic physics principles underlying the methodologies followed. © 2001 Elsevier Science Ireland Ltd. All rights reserved.