  This paper discusses low mass, metal poor stars, presenting new evolutionary
computations which take into account the inward diffusion of both He and heavy
elements during the H burning phase. The investigation has been extended to He
burning models, discussing the luminosity of the Zero Age Horizontal Branch
(ZAHB) models originated from progenitors with efficient element diffusion. We
find that diffusion of heavy elements plays a non-negligible role in the
evolutionary paths of the model. One finds that element diffusion has a
sensitive influence on evolutionary models, a less significant effect on the
isochrone turn off and, finally, a rather marginal impact on the difference in
luminosity between the ZAHB and the isochrone Turn-Off (TO). The consequences
of these results on current evaluations of globular cluster ages are discussed
in connection with possible sources of errors.
