Electroencephalography (EEG) and magnetoencephalography (MEG) provide signals that are weighted integrals of source currents in the brain. In addition to technical aspects, the two methods differ in their sensitivities to various cerebral sources. Moreover, it is more difficult to determine the lead fields of EEG than of MEG. If it can be assumed that only one localized source is active at a particular time, the source location, direction, and amplitude can be found with the dipole model. However, if the assumption of a single localized source is violated, erroneous results are obtained. If a few sources are responsible for the measured fields, multiple-dipole models can be used. In the general case one must start from the fundamentals of estimation theory. The use of a priori information, together with experimental data, will provide the best possible solution to the inverse problem. In the case of minimal prior information, the so-called minimum-norm solution is obtained. With the help of supplementary information, the resolution can be further improved.