The universe is transparent to GeV photons, making this part of the
spectrum ideal for obtaining information on high-energy astrophysical
phenomena at cosmological distances. The GeV sky was first studied in
detail by the Energetic Gamma-Ray Experiment Telescope (EGRET) in the
1990s. Together with an impressive array of data on high-energy systems,
the legacy of the EGRET mission includes two persisting puzzles: the
nature of 170 yet unidentified gamma-ray sources, and the origin of a
diffuse, isotropic, persumably extragalactic gamma-ray background.
The launch of the Gamma-ray Large Area Space Telescope in the fall of
2007 will once again open the GeV-window of energies to observations.
I will discuss the observational prospects for GLAST, together with
recent progress, using model-independent approaches, on the
interconnected questions of unidentified sources and the gamma-ray
background. Finally, I will outline additional tests with GLAST
which will enable us to further constrain relevant theories.