Functional Magnetic Resonance Imaging (fMRI) is a recent MRI technique capable of visualising neuronal activity in humans in a non-invase way. The technique visualises the physiological changes in oxy- and deoxyhemoglobin concentration changes in small cortical blood vessels upon neuronal activation without the need for radiation or the administration of contrast media or radioactive tracers. The spatial accuracy of the technique is of the order of millimeters and the temporal resolution of the order of one second. The concept has captured the interest of neuroradiologists as well as neuroscientists, who now have a means to visualise their theories in human volunteers. In the clinical environment the non-invasive studies should aid neurosurgeons in adopting a safe course into the brain and assist neurologists in unraveling neurological hypotheses. This report describes the technical principals of fMRI and presents some of our clinical results on the mapping of several cortical functions, such as motor, auditory and language functions, in a large group of patients.