The advent of powerful gradient systems and ultrafast echo-planar imaging (EPI) offers the opportunity to use magnetic resonance imaging to measure and to localize brain function with high spatio-temporal resolution in clinically feasible scanning times. Functional magnetic resonance imaging (fMRI) opens up a new diagnostic field in radiology and neuroradiology with a change from pure morphological brain imaging to the measurement and visualization of brain function. Despite its successful application in neurosurgical, neurological, psychiatric, neuropediatric and pain patients fMRI has not yet reached the status of an established clinical diagnostic procedure. To this end special stimulation systems, standardized fMRI protocols and medically approved software, all dedicated to clinical application, are necessary. The training and teaching of doctors and radiographers will also be crucial for the progress of clinical fMRI. This paper gives an overview of the neurophysiological background, the technical requirements and the data processing strategies that are relevant for the clinical application of fMRI. Presurgical fMRI in patients with brain tumors is used as an example for the clinical relevance of the method.