Magnetic fields in spiral galaxies

The methods and results of radio and optical studies of galactic magnetic fields are examined. Field strengths based on the minimum energy condition for sources of synchrotron radiation range from about 3 μgauss to 15 μgauss with the most commonly observed field geometry being the bisymmetric spiral. Current understanding of galactic dynamos driven by differential rotation and helical motions of ionized gas is briefly discussed. (author)