  Astrophysical jets are associated with the formation of young stars of all
masses, stellar and massive black holes, and perhaps even with the formation of
massive planets. Their role in the formation of planets, stars, and galaxies is
increasingly appreciated and probably reflects a deep connection between the
accretion flows - by which stars and black holes may be formed - and the
efficiency by which magnetic torques can remove angular momentum from such
flows. We compare the properties and physics of jets in both non-relativistic
and relativistic systems and trace, by means of theoretical argument and
numerical simulations, the physical connections between these different
phenomena. We discuss the properties of jets from young stars and black holes,
give some basic theoretical results that underpin the origin of jets in these
systems, and then show results of recent simulations on jet production in
collapsing star-forming cores as well as from jets around rotating Kerr black
holes.
