  The COMPTEL and EGRET detectors aboard the Compton Gamma-Ray Observatory
measured an extragalactic gamma-ray background extending from MeV energies up
to about 100 GeV. Calculations performed making reasonable assumptions indicate
that blazars can account for the background between about 10 MeV and at least
10 GeV. Below 30 MeV, the background flux and spectrum are not very well
determined and a dedicated satellite detector will be required to remedy this
situation. Below 10 MeV, supernovae and possibly AGN may contribute to the
extragalactic background flux. Above 10 GeV, the role of blazars in
contributing to the background is unclear because we do not have data on their
spectra at these energies and because theoretical models predict that many of
them will have spectra which should cut off in this energy range. At these
higher energies, a new component, perhaps from topological defects, may
contribute to the background, as well as X-ray selected BL Lac objects. The
future GLAST detector should provide important data on the emission of
extragalactic sources above 10 GeV and help resolve this issue. GLAST may also
be able to detect the signature of intergalactic absorption by pair production
interactions of background gamma-rays of energy above 20 GeV with starlight
photons, this signature being a steepening of the background spectrum.
