  During this last decade our knowledge of the evolutionary properties of stars
has significantly improved. This result has been achieved thanks to our
improved understanding of the physical behavior of stellar matter in the
thermal regimes characteristic of the different stellar mass ranges and/or
evolutionary stages. This notwithstanding, the current generation of stellar
models is still affected by several, not negligible, uncertainties related to
our poor knowledge of some thermodynamical processes and nuclear reaction
rates, as well as the efficiency of mixing processes. These drawbacks have to
be properly taken into account when comparing theory with observations, to
derive evolutionary properties of both resolved and unresolved stellar
populations. In this paper we review the major sources of uncertainty along the
main evolutionary stages, and emphasize their impact on population synthesis
techniques.
