  Classical novae produce radioactive nuclei which are emitters of gamma-rays
in the MeV range. Some examples are the lines at 478 and 1275 keV (from 7Be and
22Na) and the positron-electron annihilation emission (511 keV line and a
continuum below this energy, with a cut-off at 20-30 keV). The analysis of
gamma-ray spectra and light curves is a potential unique and powerful tool both
to trace the corresponding isotopes and to give insights on the properties of
the expanding envelope determining its transparency. Another possible origin of
gamma-rays is the acceleration of particles up to very high energies, so that
either neutral pions or inverse Compton processes produce gamma-rays of
energies larger than 100 MeV. MeV photons during nova explosions have not been
detected yet, although several attempts have been made in the last decades; on
the other hand, GeV photons from novae have been detected in some particular
novae, in symbiotic binaries, where the companion is a red giant with a wind,
instead of a main sequence star as in the cataclysmic variables hosting
classical novae. Both mechanisms of gamma-ray production in novae are reviewed,
with more emphasis on the one related to radioactivities.
