I am particularly interested in the application of ground-based air shower detectors for gamma-ray astronomy. During the past decade, the unanticipated detection of high energy gamma-rays (between energies of 1 GeV and 10 TeV) from dozens of sources by space- and ground-based experiments has revolutionized our understanding of high energy processes in the universe. Gamma-rays result from high-energy interactions in the most compact and energetic sources inside and outside our galaxy. Gamma-rays from pulsars and supernova remnants are providing critical clues to the origin of cosmic rays. Furthermore gamma-rays dominate the outburst emission of certain AGN galaxies -- called "Blazars" -- indicating the presence of relativistic jets beamed toward the observer. Jets also appear to play a critical role in explaining the mysterious gamma-ray burst sources. Observed absorption features in extra-galactic gamma-ray spectra may provide an important probe of the intergalactic IR and optical photon field which in turn places significant constraints on cosmological models and theories of structure formation. A complete understanding of the high energy processes occurring in these most energetic sources requires full-spectrum gamma-ray observations. Until recently, the energy range from 20 to 300 GeV has been inaccessible, representing a gap in the gamma-ray spectrum and hindering further progress in gamma-ray astronomy.