  Observational evidence for dynamo action in spiral galaxies is reviewed, and
the capabilities of various theories in explaining the basic features of
galactic magnetic fields are discussed. Mean-field dynamo models appear to be
unique in providing a coherent explanation of a wide variety of magnetic
features in spiral galaxies.
  Reliable modelling of global magnetic structures, such as the magnetic ring
in M31, requires detailed knowledge of the rotation curve, the magnitude and
radial profile of turbulent and noncircular systematic velocities, the scale
height of the warm ionized layer, the total gas density, the turbulent scale
and their variations with galactocentric radius. More detailed models involving
the effects of the spiral arms on magnetic field require the knowledge of the
arm-interarm contrasts in the above quantities.
