  We review the wide range of astrophysical observations of gamma ray emission
lines and we discuss their implications. We consider line emission from solar
flares, the Orion molecular cloud complex, supernovae 1987A and 1991T, the
supernova remnants Cas A and Vela, the interstellar medium, the Galactic center
region and several Galactic black hole candidates. The observations have
important, and often unique, implications on particle acceleration, star
formation, processes of nucleosynthesis, Galactic evolution and compact object
physics.
