Radio observations of the continuum emission are best suitable to study the magnetic fields in galaxies. The total intensity of the synchrotron emission gives the strength of the total magnetic field. The linearly polarized intensity reveals the strength and the structure of the resolved regular field in the plane of the sky. However, the observed polarization vectors suffer Faraday rotation and depolarization (i.e. a decrease of the degree of linear polarization when compared to the intrinsic one) on the way from the radiation's origin to us. Correction for Faraday rotation is possible with observations at two or better more wavelengths by determining the rotation measure RM (being proportional to ne B|| dl). The rotation measure itself is a measure of the magnetic field strength parallel to the line of sight, whereas its sign gives the direction of this magnetic field component. The field strength of both components, parallel and perpendicular to the line of sight, together with the information of the intrinsic polarization vectors enables us in principle to perform a `tomography' of the magnetic field.