The fitting of isochrones over star-cluster colour-magnitude diagrams is a favourite test of stellar evolution models because it seems to be the easiest and most evident one: the star distribution in the colour-magnitude diagram represents a constant age locus, provided the clusters are old enough and the internal age dispersion is negligible. The ratios of evolved to main-sequence stars in open clusters, or of population along the red giant branch in globular clusters, could provide valuable tests, but it is difficult to estimate these ratios with small enough uncertainties. Furthermore the detailed surface chemical composition (element abundances and isotope ratios) is not known for many cluster stars and model predictions cannot be tested in various environments.

Tests have been mostly devoted to the main sequence and the long debate between canonical precepts and core overshooting is still active. St others (1991) has argued that the use of the new opacities (Rogers & Iglesias 1992) reduces the necessity of adding overshooting to the models. Castellani et al (1992) computed extensive grids of models with canonical precepts. However, recent papers (Carraro et al. 1993; Meynet et al. 1993; Demarque et al. 1994) show that a moderate core overshooting (about 0.2 Hp) is still necessary. Basically, cluster observations call for more massive cores, whatever process is at work. An excellent review of the various ingredients included in models (equation of state, nuclear rates, opacities, convection) and the effects of their changes has been presented by Vandenberg (1991).