Prior to the launch of CGRO, a number of observations of the Galactic
diffuse emission were made with instruments
covering generally non-overlapping energy ranges. At MeV energies the
-ray spectrometer on SMM
(Harris et al. 1990)
has provided high resolution spectra of the Galactic emission with
little spatial information. At higher energies above 50 MeV SAS-2
and COS-B have observed the Galactic plane with better spatial resolution, but
moderate energy resolution (for a review see
Bloemen 1989).
The intermediate energy range remained uncharted.
More recently the instruments OSSE, COMPTEL, and EGRET on CGRO have
provided us with a wealth of data of the Galactic diffuse -ray emission,
and thus with a much clearer understanding of the spectrum and the
spatial distribution of the line and continuum radiation. In the next
sections I will examine the main results of these observations, particularly
those which fail to conform with the standard picture. Finally I shall
discuss the constraints for cosmic ray physics thus derived.