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.