Brain functional MRI (fMRI) provides an indirect mapping of cerebral activity, based on the detection of the local blood flow and oxygenation changes following neuronal activity (BOLD contrast, Blood Oxygenation Level Dependent). fMRI allows us to study non invasively the normal and pathological aspects of cortical functional organization. Each fMRI study compares two different states of activity. Echo-Planar Imaging (EPI) is the technic of choice that makes it possible to study the whole brain at a rapid pace. Activation maps are calculated from a statistical analysis of the local signal changes. Functional MRI is now becoming an essential tool in the neurofunctional work-up of many neurosurgery patients, as well as the reference method to image normal or pathologic functional brain organization in adults and children.