  Cosmology, high-energy physics and astrophysics are converging on the study
of large-scale magnetic fields. While the experimental evidence for the
existence of large-scale magnetization in galaxies, clusters and superclusters
is rather compelling, the origin of the phenomenon remains puzzling especially
in light of the most recent observations. The purpose of the present review is
to describe the physical motivations and some of the open theoretical problems
related to the existence of large-scale magnetic fields.
