We define astrophysical jets as linear structures detected in the sky, typically bi-polar, and originating from a common source. 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. 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.
Recent observations by the neutrino detector system, ICECUBE, in concert with coordinated observations by the MAGIC Collaboration Cerenkov telescopes and the Fermi LAT, have for the first time given us direct evidence that the jets consist of hadronic elements. Remarkably, the recent observations of gold line emission produced by the kilonova associated with gravitational waves (see, e.g. Seigel, 2019) also suggest that jets form even duringgravitational wave generation during compact NS-NS object coalescence.