  One of the observational evidences in support of the "thermonuclear runaway
model" for the classical nova outburst relies on the accompanying
nucleosynthesis. In this paper, we stress the relevant role played by
nucleosynthesis in our understanding of the nova phenomenon by constraining
models through a comparison with both the atomic abundance determinations from
the ejecta and the isotopic ratios measured in presolar grains of a likely nova
origin. Furthermore, the endpoint of nova nucleosynthesis provides hints for
the understanding of the mixing process responsible for the enhanced
metallicities found in the ejecta, and reveals also information on the
properties of the underlying white dwarf (mass, luminosity...).
  We discuss first the interplay between nova outbursts and the Galactic
chemical abundances: Classical nova outbursts are expected to be the major
source of 13C, 15N and 17O in the Galaxy, and to contribute to the abundances
of other species with A < 40, such as 7Li or 26Al. We describe the main nuclear
path during the course of the explosion, with special emphasis on the synthesis
of radioactive species, of particular interest for the gamma-ray output
predicted from novae (7Li, 18F, 22Na, 26Al). An overview of the recent
discovery of presolar nova candidate grains, as well as a discussion of the
role played by nuclear uncertainties associated with key reactions of the
NeNa-MgAl and Si-Ca regions, are also given.
