The ability to accurately map functional cortex both preoperatively and intraoperatively is an important neurosurgical challenge for the next decade. The central concept of preoperative mapping is to superimpose blood flow and electrical activation data on a three-dimensional matrix created by MR imaging and image processing. The gold standard, at present, is direct cortical stimulation, which can identify primary motor, sensory, speech, and visual cortices. Cortical mapping can be performed intraoperatively or through implanted subdural electrodes at the bedside. The tasks for the next decade include the validation of these preoperative techniques (fMRI, MEG, TMS, EP): 1. Correlation of direct cortical stimulation with advanced perioperative stimulation/monitoring systems, in a wide variety of cases. In addition, extension of these applications into intraoperative imaging systems like the GE Signa unit should also be envisaged. 2. Evaluation of how various pathologies such as AVMs, well-circumscribed tumors, and infiltrative tumors affect the data obtained by these techniques. 3. Expansion of mapping to areas outside the primary motor and sensory cortices, including those referred to as "associative" and to higher functions collectively termed as "cognition."