  The abundances of the isotopes of the elements C, N and O are mainly affected
by the cold CNO cycles in non-explosive stellar situations, or by the hot CNO
chains that can develop in certain explosive sites, like classical novae.
Helium burning phases can modify the composition of the ashes of the CNO
transmutations through several $\alpha$-capture reactions, the most famed one
being 12C(a,g)16O. This contribution presents a short review of the purely
nuclear physics limitations imposed on the accuracy of the predicted C, N and O
yields from H-burning in non-explosive stars or novae. This analysis makes
largely use of the NACRE compilation for the rates of the reactions on stable
targets making up the cold CNO cycle. Some more recent rate determinations are
also considered. The analysis of the impact of the rate uncertainties on the
abundance predictions is conducted in the framework of a simple parametric
astrophysical model. These calculations have the virtue of being a guide in the
selection of the nuclear uncertainties that have to be duly analyzed in
detailed model stars, particularly in order to perform meaningful
confrontations between abundance observations and predictions. They are also
hoped to help nuclear astrophysicists pinpointing the rate uncertainties that
have to be reduced most urgently. A limited use of detailed stellar models is
also made for the purpose of some specific illustrations.
