Recent developments in the use of Magnetic Resonance Imaging (MRI) to measure and image molecular diffusion and blood microcirculation (perfusion) hold significant promise in the noninvasive evaluation of normal brain function and functional disorders. Molecular diffusion is the result of spontaneous random motion that involves all molecules and probes molecular motion at microscopic level. Using diffusion MRI, information on tissue geometry and compartmentation effects can be obtained. Diffusion MRI has been used to map myelin fiber orientation in brain with high accuracy. Diffusion MRI is also the only imaging modality which shows brain ischemia at a very early stage, even before T1w or T2w MR images become abnormal, offering great promises in the management of stroke patients. Also, diffusion imaging may be used to monitor tissue temperature changes noninvasively during hyperthermia or laser surgery. On the other hand, MRI can provide information on tissue perfusion. Several methods have been proposed, some of them including tracers or contrast agents. The most successful approach for brain function studies, however, is based on the sensitivity of MRI to magnetic effects induced by changes in the oxygenation status of hemoglobin (deoxyhemoglobin). These effects have already been used to characterize hematomas. These effects may also be exploited to detect small modulation in red blood cell oxygen content related to local variations in blood flow and oxygen consumption in tissues. In the brain cortex, such variations may be induced by external stimuli or internal cognitive processes. Capillary blood deoxyhemoglobin thus acts as a natural endogeneous contrast agent.(ABSTRACT TRUNCATED AT 250 WORDS)