Atmospheric Cerenkov telescopes have, somewhat surprisingly, turned out to be the most sensitive probes of activity in jets emerging from Active Galactic Nuclei (AGNs). Whipple Observatory observations of Mrk 421, Mrk 501, 1ES 2344+514 and H1426+428 together with EGRET observations of 3C 279, have become the benchmarks for AGN emission theories.
TeV gamma-ray observations give a unique view of the formation of shocks at irregular intervals every few days near the base of the jet. Flares almost as short as the basic timescale associated with accretion unto a supermassive black hole have been seen, indicating that we may be observing the processes right at the beginning of the jet.
Correlated multi-wavelength observations have been most important in testing the general process of the radiation which is consistent with a synchrotron component extending up to a few keV, followed by the Compton-scattered X-ray to gamma-ray component which probably falls off at energies above 1 TeV (Buckley et al. 1996).
VERITAS will contribute enormously to the understanding of AGNs in several ways. The good flux sensitivity will enable precise measurements of the rise and fall of intensity in flares. The versatility will offer the potential for greater monitoring of AGN activity. Spectral measurements will provide important information on the processes occurring in the jets as well as on the density of the extragalactic infrared photon field through which the TeV photons pass and consequently interact, a fact which limits the visibility of distant AGNs.