Theoretical stellar models for old galactic clusters

We present new evolutionary stellar models suitable for old Population I cl usters, discussing both the consequences of the most recent improvements in the input physics and the effect of element diffusion within the stellar s tructures. Theoretical cluster isochrones are presented, covering the range of ages from 1 to 9 Gyr for the four selected choices of the metallicity Z = 0.007, 0.010, 0.015 and 0.020. Theoretical uncertainties on the efficien cy of superadiabatic convection are discussed in some detail. Isochrone fit ting to the CM diagrams of the two well-observed galactic clusters NGC 2420 and M67 indicates that a mixing-length parameter alpha = 1.9 appears adequ ate for reproducing the observed colour of cool giant stars. The problems i n matching theoretical predictions to the observed slope of MS stars are di scussed.