  We describe the properties of the blazars detected by EGRET and summarize the
results on the calculations of the evolution and luminosity function of these
sources.
  Of the large number of possible origins of extragalactic diffuse gamma-ray
emission, it has been postulated that active galaxies might be one of the most
likely candidates. However, some of our recent analyses indicate that only 25
percent of the diffuse extragalactic emission measured by SAS-2 and EGRET can
be attributed to unresolved gamma-ray blazars.
  Therefore, other sources of diffuse extragalactic gamma-ray emission must
exist.
  We present a summary of these results in this article.
