  Radio synchrotron emission, its polarization and its Faraday rotation are
powerful tools to study the strength and structure of interstellar magnetic
fields. The total intensity traces the strength and distribution of total
magnetic fields. Total fields in gas-rich spiral arms and bars of nearby
galaxies have strengths of 20-30 $\mu$Gauss, due to the amplification of
turbulent fields, and are dynamically important. In the Milky Way, the total
field strength is about 6 $\mu$G near the Sun and several 100 $\mu$G in
filaments near the Galactic Center. -- The polarized intensity measures ordered
fields with a preferred orientation, which can be regular or anisotropic
fields. Ordered fields with spiral structure exist in grand-design, barred,
flocculent and even in irregular galaxies. The strongest ordered fields are
found in interarm regions, sometimes forming "magnetic spiral arms" between the
optical arms. Halo fields are X-shaped, probably due to outflows. -- The
Faraday rotation of the polarization vectors traces coherent regular fields
which have a preferred direction. In some galaxies Faraday rotation reveals
large-scale patterns which are signatures of dynamo fields. However, in most
galaxies the field has a complicated structure and interacts with local gas
flows. In the Milky Way, diffuse polarized radio emission and Faraday rotation
of the polarized emission from pulsars and background sources show many
small-scale and large-scale magnetic features, but the overall field structure
in our Galaxy is still under debate.
