The EGRET instrument on the Compton Gamma Ray Observatory (1991-2000) has positively detected high energy gamma-ray emission from more than 67 active galaxies of the blazar class. The majority of the EGRET blazars are flat-spectrum radio quasars, which are characterized by inferred isotropic luminosities often as high as 3E49 ergs/s. The remainder are BL Lac objects, some of which have been detected at TeV energies (> 250 GeV) by ground-based atmospheric Cherenkov telescopes. One of the remarkable characteristics observed in blazars is that the gamma-ray luminosity often dominates the bolometric power in these sources. The detection of blazars by EGRET has undoubtedly been one of the highlights of the mission, and has forever impacted upon our understanding of the emission mechanisms in these objects. In this article, we summarize the properties of EGRET blazars, and review the constraints that the EGRET observations place on the various models of gamma-ray production in these sources.

Invited review paper, to appear in the proceedings of the International Symposium on High Energy Gamma-Ray Astronomy, Heidelberg, June 2000, AIP Conference Proc. 14 pages, 5 figures