  Collimated outflows (jets) appear to be a ubiquitous phenomenon associated
with the accretion of material onto a compact object. Despite this ubiquity,
many fundamental physics aspects of jets are still poorly understood and
constrained. These include the mechanism of launching and accelerating jets,
the connection between these processes and the nature of the accretion flow,
and the role of magnetic fields; the physics responsible for the collimation of
jets over tens of thousands to even millions of gravitational radii of the
central accreting object; the matter content of jets; the location of the
region(s) accelerating particles to TeV (possibly even PeV and EeV) energies
(as evidenced by gamma-ray emission observed from many jet sources) and the
physical processes responsible for this particle acceleration; the radiative
processes giving rise to the observed multi-wavelength emission; and the
topology of magnetic fields and their role in the jet collimation and particle
acceleration processes. This chapter reviews the main knowns and unknowns in
our current understanding of relativistic jets, in the context of the main
model ingredients for Galactic and extragalactic jet sources. It discusses
aspects specific to active Galactic nuclei (especially blazars) and
microquasars, and then presents a comparative discussion of similarities and
differences between them.
