  Spiral galaxies, including the Milky Way, have large-scale magnetic fields
with significant energy densities. The dominant theory attributes these
magnetic fields to a large-scale dynamo. We review the current status of dynamo
theory and discuss various numerical simulations designed to explain either
particular aspects of the problem or to reproduce galactic magnetic fields
globally. Our main conclusions can be summarized as follows. Idealized direct
numerical simulations produce mean magnetic fields, whose saturation energy
density tends to decline with increasing magnetic Reynolds number. This is
still an unsolved problem. Large-scale galactic magnetic fields of microgauss
strengths can probably only be explained if helical magnetic fields of small or
moderate length scales can rapidly be ejected or destroyed. Small-scale dynamos
are important throughout a galaxy's life, and probably provide strong seed
fields at early stages. The circumgalactic medium (CGM) may play an important
role in driving dynamo action at small and large length scales. These
interactions between the galactic disk and the CGM may provide important
insights into our understanding of galactic dynamos. We expect future research
in galactic dynamos to focus on the cosmological history of galaxies and the
interaction with the CGM as means of replacing the idealized boundary
conditions used in earlier work.
