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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 gamma-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 gamma-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.