  We recall the basic physical principles governing the evolution of stars with
some emphasis on the role played by the nuclear reactions. We argue that in
general it is not possible from observations of stars to deduce constraints on
the nuclear reaction rates. This is the reason why precise measurements of
nuclear reaction rates are a necessity in order to make progresses in stellar
physics, nucleosynthesis and chemical evolution of galaxies. There are however
some stars which provides useful constraint on nuclear processes. The
Wolf-Rayet stars of the WN type present at their surface CNO equilibrium
patterns. There is also the particular case of the abundance of $^{22}$Ne at
the surface of WC stars. The abundance of this element is a measure of the
initial CNO content. Very interestingly, recent determinations of its abundance
at the surface of WC stars tend to confirm that massive stars in the solar
neighborhood have initial metallicities in agreement with the Asplund et al.
(2005) solar abundances.
