  Various types of magnetic fields occur in stars: small scale fields, large
scale fields, and internal toroidal fields. While the latter may be ubiquitous
in stars due to differential rotation, small scale fields (spots) may be
associated with envelop convection in all low and high mass stars. The stable
large scale fields found in only about 10 per cent of intermediate mass and
massive stars may be understood as a consequence of dynamical binary
interaction, e.g., the merging of two stars in a binary. We relate these ideas
to magnetic fields in white dwarfs and neutron stars, and to their role in
core-collapse and thermonuclear supernova explosions.
