The strength and structure of interstellar magnetic fields can be well
studied by observations of radio continuum emission, and its polarization
and Faraday rotation. Fields with a well-ordered spiral structure exist
in grand-design, barred, flocculent and even in irregular galaxies. Total
field strengths in spiral arms and bars are 10-30 MicroGauss, due to
compression of turbulent fields, and are dynamically important. The
strongest regular fields are found in interarm regions, sometimes forming
``magnetic spiral arms'' between the optical arms. Faraday rotation of
the polarization vectors sometimes reveals large-scale patterns which are
signatures of coherent fields generated by dynamos. However, in most
galaxies the field have complicated structures and interact with local
gas flows.