ABSTRACT
The strongest, and hence first, gamma-ray source to be detected (see historical note: first light), the galactic plane remains one of the prime objects for study in gamma-ray astrophysics and one of its major contributions to astrophysics. That this should be so is attributable to the great difficulty that the general study of the plane presents (because of our immersion in it) and one of the great properties of cosmic gamma rays, their ability to penetrate interstellar matter. The study of the diffuse component of the gamma-ray flux from the Galaxy provides a powerful tool to study the cosmic radiation in the Galaxy as a whole. The galactic plane is unique in that it is the only cosmic source thus far detected where one can unequivocally identify hadrons as the progenitors for at least part of the observed spectrum. However, in a sense the galactic plane is not a source but a medium through which cosmic rays propagate and, hence, the true source of hadronic cosmic rays remains elusive.
