  The uncertainties in the modelling of some important inputs of stellar
evolution must be taken into account for a correct interpretation, both of the
HR diagrams of individual stars, and of the integrated colors, of stellar
clusters. After a short discussion of Globular Cluster ages, we focus on the
problem of convection and discuss the parametrization of convection efficiency
and overshooting. Convective efficiency affects the reliability of the giants
colors, especially for high metallicity: age-metallicity relations found from
integrated colors of clusters must be regarded with caution. Non-instantaneous
mixing, both in the formal convective region and in the overshooting region
must be taken into account: this is necessary to compute nucleosynthesis in
`hot bottom burning' envelopes, but it may also affect the color distribution
in clump stars as shown here for the case of the LMC cluster NGC 1866.
