The observed radio-continuum in galaxies is the addition of thermal and non-thermal emissions. Thermal emission, due to hot gas ionized by stars, has a flat spectrum and is best observed at high radio frequencies. The non-thermal emission, due to relativistic electrons spiralling in magnetic fields of a galaxy, has a steep spectrum and dominates at lover radio frequencies. Relativistic electrons can originate in supernovae, pulsars, X-ray sources and CB-stars. The non-thermal emission process offers an additional possibility since the radio emission is linearly polarized. We can use the observations of polarized radiation to trace the magnetic fields in galaxies.