ABSTRACT
From the cosmologist’s perspective, there is no aspect of gamma-ray astronomy as important as the truly diffuse gamma-ray background. The extremely good penetrating power of HE gamma rays raises the prospect of gamma rays coming from the earliest times (out to z = 100) and, hence, giving a glimpse of conditions at cosmological times that is otherwise not available. VHE gamma rays are of little use in this regard since the techniques available do not permit the detection of a diffuse background and are unlikely to do so in the future. Also VHE gamma rays do not have the penetrating power of HE gamma rays-ironically this can be used to provide unique information on the cosmic infrared background out to distances of z = 1 (see later).
