This Assessment examines the available evidence to determine whether magnetoencephalography (MEG) and/or magnetic source imaging (MSI) provide additional diagnostic information that improves the management and outcomes of patients who are being evaluated for neurosurgical treatment in two main clinical uses. The first group includes patients being evaluated for surgical treatment of epilepsy, including those with seizures refractory to medical management. MEG/MSI would be used to characterize the location of the epileptic zone for resection in the hopes of either identifying additional surgical candidates or avoiding the need for invasive confirmatory testing. The second group includes patients who are undergoing presurgical evaluation for removal of a brain lesion, for which mapping of various functional anatomical areas is necessary.