Brain regions are removed to treat lesions, but great care must be taken not to disturb or remove functional areas in the lesion and in surrounding tissue where healthy and diseased cells may be intermingled, especially for infiltrating tumors. Cortical functional areas and fiber tracts can be localized preoperatively by probabilistic anatomical tools, but mapping of functional integrity by neurophysiology is essential. Identification of the primary motor cortex seems to be more effectively performed with transcranial magnetic stimulation (TMS) than functional magnetic resonance imaging (fMRI). Language area localization requires auditory evoked potentials or TMS, as well as fMRI and diffusion tensor imaging for fiber tracts. Somatosensory cortex is most effectively mapped by somatosensory evoked potentials. Crucial eloquent areas, such as the central sulcus, primary somatomotor areas, corticospinal tract must be defined and for some areas that must be removed, potential compensations may be identified. Oncological/functional ratio must be optimized, resecting the tumor maximally but also sparingly, as far as possible, the areas that mediate indispensable functions. In some cases, a transient postoperative deficit may be inevitable. In this article, we review intraoperative exploration of motricity, language, somatosensory, visual and vestibular function, calculation, memory and components of consciousness.