  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
0.6Mo and 1.4Mo. 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.
