  We discuss theoretical predictions concerning the evolution of globular
cluster Pop.II stars vis-a-vis current estimates of standard errors in the
determination of nuclear burning rates. Numerical evaluations are given for the
dependence of TO and HB luminosity from the rate of the relevant nuclear
reactions. We conclude that evolutionary predictions appear rather solid in
this respect, with a maximum 3$\sigma$ error of about 1 Gyr in the evaluation
of cluster ages derived from the calibration of the difference in luminosity
between HB and TO. However, current evaluation of the original He content, as
given on the basis of the R parameter, should wait for a much better
determination of the C12($\alpha$,$\gamma$)O16 reaction before reaching a
satisfactory accuracy.
