  Novae and supernovae play a key role in many fields of Astrophysics and
Cosmology. Despite their importance, an accurate description of which objects
explode and why and how they explode is still lacking. One of the main
characteristics of such explosions is that they are the main suppliers of newly
synthesized chemical elements in the Galaxy. Since some of these isotopes are
radioactive, it is possible to use the corresponding gamma-rays as a diagnostic
tool of the explosion thanks to their independence on the thermal state of the
debris. The drawback is the poor sensitivity of detectors in the MeV energy
domain. As a consequence, the radioactive lines have only been detected in one
core collapse supernova (SN 1987A), one Type Ia supernova (SN 2014J), and one
supernova remnant (Cas A). Nevertheless these observations have provided and
are providing important information about the explosion mechanisms.
Unfortunately, novae are still eluding detection. These results emphasize the
necessity to place as soon as possible a new instrument in orbit with enough
sensitivity to noticeably enlarge the sample of detected events
