We will use magnetoencephalography (MEG) alone, and in combination with EEG (MEG-EEG), to study the contribution of each method and their combination to the localizing yield of the non-invasive pre-surgical evaluation as compared to the invasive method. We will also correlate seizure origin and localizing data with surgical outcome, and, in retrospect, calculate the proportion of patients in whom invasive monitoring could have been avoided.