The beautiful images of extragalactic radio jets obtained with the VLA (see Laing's paper in these proceedings) demonstrate that enormous amounts of energy in exotic forms are transported from the nuclei of radio galaxies and quasars to interstellar and intergalactic space. These beams of highly relativistic particles and magnetic field have remarkably narrow opening angles of only a few degrees. At milliarcsecond resolution, using VLBI, we find parsec-scale jets just as highly collimated, with features that not only move outward, but do so at speeds that appear to exceed that of light.
It is therefore apparent that something deep within the nucleus of the active galaxy generates relativistic, magnetized plasma, directs the plasma into an outward flow, and focuses the flow into a narrow cone. The main goal of studying compact jets is to determine how the plasma is energized, accelerated to relativistic flow speeds, and collimated into a thin jet. It is hoped that an understanding of these processes will also shed light on the nature of the central engine that powers the nuclear activity in the galaxy or quasar.
There are two basic observational methods for studying compact jets. The first is direct imaging using VLBI. The second is to follow time variations in flux density and polarization at different wavebands. These two techniques are combined when one uses VLBI to monitor structural changes with time.
Fourteen years ago, Blandford & Rees (1978) proposed that variable extragalactic radio sources can be explained by relativistic jets, defined as collimated, gas dynamical flows containing collisionless relativistic plasmas threaded by a dynamically unimportant magnetic field and moving at bulk velocities close to the speed of light. Later that year, Readhead et al. (1978) published VLBI images using new hybrid mapping techniques which improved the dynamic range and fidelity considerably. These maps revealed that the compact radio sources observed consist of a compact "core" plus a one-sided series of knots that trace out a jet, thereby supporting the relativistic jet model. This model survives to the present day and has received support from a number observations that would be difficult to interpret in other ways.
In what follows, I will review the observations of compact extragalactic jets, relating the data to the relativistic jet scenario wherever applicable. I will also discuss speculative models that connect the jets on the scales observed with VLBI to the region immediately surrounding the central engine, whose main influence on the nonthermal source is limited to scales roughly two orders of magnitude smaller than can be directly imaged. Finally, I will indicate how further observations, especially multiwavelength monitoring, can test such models.