The nuclei of most normal galaxies contain supermassive black holes, which can accrete gas through a disk and become active. These active galactic nuclei (AGNs) can form jets that are observed on scales from astronomical units to megaparsecs and from meter wavelengths to TeV energies. High-resolution radio imaging and multiwavelength/messenger campaigns are elucidating the conditions under which this happens. Evidence is presented that:
Relativistic AGN jets are formed when the black hole spins and the the accretion disk is strongly magnetized, perhaps on account of gas accreting at high latitude beyond the black hole sphere of influence.
AGN jets are collimated close to the black hole by magnetic stress associated with a disk wind.
Higher-power jets can emerge from their galactic nuclei in a relativistic, supersonic, and proton-dominated state, and they terminate in strong, hot spot shocks; lower-power jets are degraded to buoyant plumes and bubbles.
Jets may accelerate protons to EeV energies, which contribute to the cosmic ray spectrum and may initiate pair cascades that can efficiently radiate synchrotron γ-rays.
Jets were far more common when the Universe was a few billion years old and black holes and massive galaxies were growing rapidly.
Jets can have a major influence on their environments, stimulating and limiting the growth of galaxies.
The observational prospects for securing our understanding of AGN jets are bright
