Two abnormalities in the spontaneous magneticbrain activity of patients with tumours and epilepsy areinvestigated: slow waves and inter-ictal spikes. Signalanalysis techniques are demonstrated to localise theirunderlying generators, project these sources onto thepatient’s MR scan and to compare their locations. Theclinical importance of this study is to get insight into thequestion whether the (abundant) slow waves can be used as adiagnostic substitute for the (relatively rare) inter ictalspikes. ; Two abnormalities in the spontaneous magneticbrain activity of patients with tumours and epilepsy areinvestigated: slow waves and inter-ictal spikes. Signalanalysis techniques are demonstrated to localise theirunderlying generators, project these sources onto thepatient’s MR scan and to compare their locations. Theclinical importance of this study is to get insight into thequestion whether the (abundant) slow waves can be used as adiagnostic substitute for the (relatively rare) inter ictalspikes.