  The jets of active galactic nuclei can carry a large fraction of the accreted
power of the black-hole system into interstellar and even extragalactic space.
They radiate profusely from radio to X-ray and gamma-ray frequencies. In the
most extreme cases, the outward flow speeds correspond to high Lorentz factors
that can reach 40 or more. This chapter displays images at various wavebands as
well as light curves and continuum spectra that illustrate the variability with
location, time, and frequency of the emission from compact, parsec- and
subparsec-scale jets. It presents a physical framework for investigating many
aspects of the structure and dynamical processes from such data.
