This review is part of a series that considers our evolving understanding of the nature of astrophysical jets. These have been defined as linear structures on the sky, typically bi-polar, and originating from a common source. Jets are associated with outflows originating from accretion processes in star-forming regions,compact objects (such as neutron stars and black holes), \gamma-ray bursts associated with stellar collapse, active galactic nuclei, and in some pulsars. In pulsars, the jet production can be drawn from angular momentum loss from the pular. 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 and Doppler boosted. This phenomenon is usually interpreted as evidence of bulk relativistic motion of the emitting particles. 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. In addition, the degree of international cooperation required for observing campaigns of these sorts is remarkable, since the instruments include among others the Very Large Array (VLA), the Very Long Baseline Array (VLBA), and entire constellations of satellite instruments, often working in concert. This last point is critically important. The increased resolution of observational systems has taken us from degrees to arcminutes to arcseconds to milliarcseconds, and now to microarcseconds in certain instances. The milliarscecond and microarcsecond observations have shown us the truly dynamic nature of the emitting regions of these remarkable objects. In this paper, I discuss some relevant observations from these efforts and the theoretical interpretations they have occasioned.