  The most energetic part of the electromagnetic spectrum bears the purest
clues to the synthesis of atomic nuclei in the universe. The decay of
radioactive species, synthesized in stellar environments and ejected into the
interstellar medium, gives rise to specific gamma ray lines. The observations
gathered up to now show evidence for radioactivities throughout the galactic
disk, in young supernova remnants (Cas A, Vela), and in nearby extragalactic
supernovae (SN 1987A, SN 1991T and SN1998bu), in the form of specific gamma ray
lines resulting, respectively, from the radioactive decay of 26Al, 44Ti and
56Co. The various astrophysical sites of thermal nucleosynthesis of the
radioactive nuclei were discussed: AGB and Wolf-Rayet stars, novae, and type Ia
and type II supernovae. Nuclear excitations by fast particles also produce
gamma ray lines which have been observed in great detail from solar flares, and
more hypothetically from active star forming regions where massive supernovae
and WR stars abound. This non thermal process and its nucleosynthetic
consequences was reviewed. The 511 keV line arising from e+ + e- annihilation
also provides important information on explosive nucleosynthesis, as well as on
the nature of the interstellar medium where the positrons annihilate. INTEGRAL,
the main mission devoted to high resolution nuclear spectroscopy, should lead
to important progress in this field.
