Multi-frequency radio polarization observations of external galaxies are now available. The resolution is such that we can sample many positions over the galaxy, and different scale sizes. Faraday rotation measures, initial polarization angles, and depolarization can be used as indicators of magnetic field morphology in the source. In order to interpret the variation of polarization characteristics with frequency, a numerical model of Faraday rotation in galaxies is being developed. Preliminary results show that a wide range of situations can be investigated with the model.