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Published online by Cambridge University Press: 04 August 2017
Consideration of the many observed types of jets on scales ranging from parsecs to megaparsecs seen in radio, optical, infrared and X-ray wavebands with a variety of morphologies both in galactic and extragalactic systems leads to some constraints on their fundamental nature. Jet formation is introduced with the concept of the Laval nozzle and related points include the problem of maintaining the nozzle, Mach disk effects due to under and over-expansion and the potential importance of magnetic confinement and focussing. Current ideas on jet formation at the black hole and accretion disk are given with emphasis on the plasma physics associated with black-hole electrodynamics, thermal and magnetically driven winds and thick disks. Stability of jet propagation is reviewed with emphasis on magnetised and unmagnetised Kelvin-Helmholtz instabilities and the various dominant modes. The particle acceleration physics of shocks, wave-particle interactions and turbulence is summarised while noting some outstanding plasma physics problems. Jet equilibrium associated with the non-linear saturation of instabilities, the formation of cocoons, shock stabilisation and magnetic fields is discussed. Detailed plasma physics studies that could significantly clarify jet physics are indicated.