Uncertainties and Systematics in Stellar Evolution Models

In this last decade, our knowledge of evolutionary and structural properties of stars of different mass and chemical composition is significantly improved. This result has been achieved as a consequence of our improved capability in understanding and describing the physical behavior of stellar matter in the different thermal regimes characteristic of the different stellar mass ranges and/or evolutionary stages. This notwithstanding, current generation of stellar models is still affected by significant and, usually, not negligible uncertainties. These uncertainties are related to our poor knowledge of some physical procceses occurring in the real stars such as, for instance, some thermodynamical processes, nuclear reaction rates, as well as the efficiency of mixing processes. These drawbacks of stellar models have to be properly taken into account when comparing theory with observations in order to derive relevant information about the properties of both resolved and unresolved stellar populations. In this paper we review current uncertainties affecting low-mass stellar models , i.e. those structures with mass in the range between 0.6M ⊙ and ∼ 1.4M ⊙. We show what are the main sources of uncertainty along the main evolutionary stages, and discuss the present level of agreement between theory and observations concerning some selected features of the Color-Magnitude Diagram of low-mass stars.