  Classical novae are potential gamma-ray emitters, because of the
disintegration of some radioactive nuclei synthesized during the explosion.
Some short-lived isotopes (such as 13N and 18F), as well as the medium-lived
22Na, decay emitting positrons, which annihilate with electrons and thus are
responsible for the prompt emission of gamma-rays from novae. This emission
consists of a 511 keV line plus a continuum between 20 and 511 keV, and is
released before the maximum in visual luminosity, i.e., before the discovery of
the nova. The main characteristics of this prompt emission, together with the
related uncertainties (both of nuclear and hydrodynamical origin, with a
particular emphasis on the influence of the envelope properties) and prospects
for detectability are analyzed in this paper.
