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).

Gamma-ray studies of rapidly rotating neutron stars, or pulsars, can broadly be divided into two categories: pulsed emission arising directly from the neutron star, or secondary emission from energetic particles injectd into the local stellar environment. Of the seven currently known TeV gamma-ray sources three are pulsar powered synchrotron nebulae: the Crab Nebula, PSR B1706-44 and Vela.

EGRET onboard CGRO has detected pulsed emission from seven young rapidly rotating neutron stars (Thompson et al. 1997). VERITAS will bridge the gap between the highest energies accessible from space and the present lowest energies accessible from the ground. In this way, VERITAS will be in the position to address the question of why pulsed emission dominates in the sub GeV domain and unpulsed emission dominates in the sub TeV domain.

The identification of a counterpart to the gamma-ray burst recorded by the Italian-Dutch X-ray satellite, Beppo-SAX, on February 28, 1997, marked a new era for the study of this phenomena. Many questions still remain concerning the underlying mechanisms. VERITAS could provide important spectral data.

Supernova remnants (SNRs) have long been considered the primary sites for the acceleration of cosmic rays up to 10³ TeV. The evidence lending support to this belief is based on several strong arguments. First, supernova blast shock are one of the few galactic sites capable of sustaining the cosmic ray population against loss by escape and nuclear interactions.