The Compton Observatory instruments have measured γ-ray lines from several individual sources and from the diffuse interstellar medium. At this late phase of CGRO’s mission, we review the astrophysical achievements by these observations of solar flare spectra, supernova lines from Co and 44Ti isotopes, 511 keV annihilation radiation from Galactic plane and bulge, and diffuse 1809 keV radioactivity emission in the Galaxy from 26Al. We briefly address other candidate lines from 7Be and 22Na nova radioactivities, and nuclear 12C and 16O de-excitation lines from Orion or the inner Galaxy. An astronomy with γ-ray lines has been established, the derived lessons suggest specific observations with the INTEGRAL observatory and other experiments of the future.