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The field of TeV gamma-ray astronomy has matured immensely in recent years and has become a viable branch of modern astronomy. It provides a unique window to the extreme non-thermal side of the universe. Many classes of astronomical systems have been detected at TeV energies. The observations have not only shed new light on the properties of the systems themselves but also on the physical processes operating in diverse astronomical settings. For instance, taken together, Blazars, microquasars, and gamma-ray bursts (though none have been detected at TeV energies yet; Atkins et al. 2005; Albert et al. 2006b; Horan et al. 2007) may offer an excellent opportunity for us to make some tangible comparisons of the processes of particle acceleration and interaction in the jets of black holes over a vast range of physical scales (from microparsecs to megaparsecs; Cui 2005). As the capability of TeV observatories improves, it is hopeful that more sources in the established classes and, more importantly, new classes of sources (e.g., GRBs, clusters of galaxies, etc.) will be detected.

The field is of equally great interest to physicists, because it has made it possible to study some of the most important questions in physics at energies much beyond the capabilities of present and future particle accelerators. Independent of theoretical scenarios, TeV observations are capable of constraining the intrinsic spectrum of emitting particles and thus casting light on the nature of the particles and on the acceleration mechanisms. TeV observations have already begun to have a serious impact on modern cosmology. They have also provided insights into such fundamental issues as dark matter, evaporation of primordial black holes, and test of Lorentz invariance. The constraints are expected to improve as the quality of data improves. In some cases, however, the challenge is to separate astronomical and physical origins of the TeV photons detected.