{Astrophysical jets have been defined as linear structures detected
in the sky, typically bi-polar, and originating from a common source. This
review continues a series that considers our evolving understanding of the
nature of astrophysical jets. Perhaps ironically, the original modeling
consisted of analytically solveable estimates of radiation processes based
on spherical symmetry due to the difficulties of analytical solutions
without such simplifications.

We now know that jets are associated with outflows originating from
accretion processes in star-forming regions, compact objects (such as
neutron stars and black holes), active galactic nuclei, and pulsars. In
pulsars, the jet production can be drawn from angular momentum loss from the
pulsar. In accreting systems, the jet production seems to be entangled
intimately with processes in the accretion disks around the compact objects.
The jets can be extremely powerful in terms both of their kinetic luminosity
and radiative signatures, and often emit radiation over the entire
electromagnetic spectrum, from radio to $\gamma$-rays. Astrophysical jets
often appear to be one-sided or having asymmetric brightness in one of the
"lobes." This phenomenon is usually interpreted as evidence of bulk
relativistic motion of the emitting particles which produces a Doppler
boosting of the approaching lobe's brightness, and a dimunition of the brightness
of the jet lobe receeding from the observer.

In the last three decades and more and as a result of observing campaigns
using detectors sensitive from radio to $\gamma$-ray energies, theoretical
models of these sources have become richer and more complex. As noted
above, models have moved from assumptions of isotropy that made analytic
calculations possible, to fully anisotropic models of emission from the jets
and their interactions with the interstellar and intra-cluster medium. Such
anisotropic calculations are only possible because we now have extensive
computational resources that can solve the rather complex emission models
that result from such anisotropies. In addition, the degree of
international cooperation required for observing campaigns of these sorts is
remarkable.

Such observations have most recently allowed us to extimate the actual
constitution of the astrophysical jets. Recent observation (see e.g.,
ICECUBE COLLABORATION (July 2018) by the neutrino detector system, Ice Cube,
in concert with coordinated observations by MAGIC Collaboration Cherenkov
telescopes and the Fermi LAT, have for the first time allowed us to give
credence to the speculation that the jets consist of hadronic elements.

In this paper, I discuss some relevant observations from these efforts and
the theoretical interpretations they have occasioned.

Keywords: astrophysical jets, active galactic nuclei, UHE
cosmic rays, quasars, microquasars