  A review is made of recent magnetohydrodynamic (MHD) theory and simulations
of origin of jets from accretion disks. Many compact astrophysical objects emit
powerful, highly-collimated, oppositely directed jets. Included are the extra
galactic radio jets of active galaxies and quasars, and old compact stars in
binaries, and emission line jets in young stellar objects. It is widely thought
that these different jets arise from rotating, conducting accretion disks
threaded by an ordered magnetic field. The twisting of the magnetic field by
the rotation of the disk drives the jets by magnetically extracting matter,
angular momentum, and energy from the accretion disk. Two main regimes have
been discussed theoretically, hydromagnetic winds which have a significant mass
flux, and Poynting flux jets where the mass flux is negligible. Over the past
several years, exciting new developments on models of jets have come from
progress in MHD simulations which now allow the study of the origin - the
acceleration and collimation - of jets from accretion disks. Simulation studies
in the hydromagnetic wind regime indicate that the outflows are accelerated
close to their region of origin whereas the collimation occurs at much larger
distances.
