  Extragalactic radio sources, including quasars, are now typically understood
as being produced by a pair of nearly symmetric, oppositely directed
relativistic jets. While some these sources span megaparsecs, and are thus the
largest physically connected structures in the universe, emitting regions
identified as jets have now been found on all scales down to fractions of a
parsec, and jets appear to be a common element of most (maybe all) types of
active galactic nuclei (AGN). We first summarize key observations of different
classes of cosmic radio jets, and describe how they may be connected.
Theoretical models for the launching and propagation of extragalactic jets are
briefly described. All of these models assume a magnetized plasma, which
typically amounts to only a small fraction of the accreted gas, is ejected from
the vicinity of a supermassive black hole. The extreme complexity of the
relevant physics has demanded numerical simulations to examine non-linear
effects on the stability of propagating jets, and some recent results from
these efforts are summarized.
