Theorists have found the field of jets to be a lucrative playground. Chimneys, funnels, tunnels, vents and nozzles evoke common structures that we know can be responsible for launching collimated flows. However, to work on cosmic scales requires alternative conditions and unfamiliar physical regimes to be explored. Severe constraints limit the number of tenable models to just a few.

There are three challenges to face: to launch, to accelerate and to collimate. Winds can be launched and accelerated relatively easily with thermal, radiative and magnetic driving from stars and accretion discs. Vast amounts of energy are released through either collapse, external heating, mass infall or nuclear fusion. Upon expansion into the surroundings, a fraction of the released energy is in some combination of released gas, radiation and magnetic field. During transport, it is converted into radially directed kinetic energy. The discovery of jets, however, indicates that the theory is far more complex than just demonstrating the existence of a driver.

This chapter will first describe pure hydrodynamical methods for driving jets as originally invoked for the extragalactic case, but now relevant to cometary and planetary jets. The observations we have so far discussed have indicated that accretion discs and jets are very strongly correlated except in the solar system. In light of this disc–jet connection, models have concentrated on the need for a rotating accretion disc and associated magnetic torques. Differences come in ascribing the origin of the magnetic fields to the disc itself or to the central star. The most developed of these models are described below in detail.