Observations of Active Galactic Nuclei with Ground-Based Cherenkov Telescopes

Imaging Atmospheric Cherenkov Telescopes (IACTs) allow us to observe Active Galactic Nuclei (AGNs) in the 100 GeV to 20 TeV energy range with high sensitivity. The TeV γ-ray observations of the nine blazars detected so far in this energy range reveal rapid flux and spectral variability on time scales of several hours, sometimes even on time scales of a few minutes. While simple synchrotron-Compton models can explain the observed non-thermal emission, alternative models which involve high-energy protons are not yet ruled out. After reviewing the status of the major IACT experiments, we describe some recent observational results and their astrophysical implications. We conclude with a discussion of possible avenues for future research.