Many compact astrophysical objects emit powerful, highly-collimated, oppositely directed jets. Included are extra galactic radio jets of active galaxies and quasars, jets from compact stars in binary systems, 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 B field by the rotation of the disk drives the jets by magnetically and centrifugally driving 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 demonstrate the formation - acceleration and some collimation - of hydromagnetic jets from accretion disks. More recently, new theoretical and computational work at LANL have shown the existence of collimated Poynting-flux outflows from accretion disks. The Poynting-flux outflows are of particular importance for the jets observed to emanate from active galactic nuclei and quasars.