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The development of techniques whereby gamma rays of energy 100 GeV and above can be studied from the ground, using indirect, but sensitive, techniques has opened up a new area of high energy photon astronomy. The most exciting result that has come from these is the detection of highly variable fluxes of TeV gamma rays from the relativistic jets in nearby AGN. The recent detection of signals from a starburst galaxy and from a radio galaxy opens the possibility that the extragalactic emission of TeV gamma rays is a ubiquitous phenomenon. Here we attempt to summarize the properties of the sources detected so far. [Copyright &y& Elsevier]

Over the past eight years, observations of BL Lacertae type objects have been an important part of the observing program at the Whipple Observatory 10 m γ-ray telescope. During this time, TeV emission was detected from five such objects: Markarian 421, Markarian 501, 1ES2344+514, H1426+428 and 1ES1959+650. Upper limits were placed on the TeV emission from 29 other BL Lacertae objects. Among these objects are two unconfirmed TeV sources, 3C66A and BL Lacertae. An overview of the results of these observations of BL Lacertae objects, between 1995 and 2000, at the Whipple Observatory is presented here. [Copyright &y& Elsevier]