  In this review, I will discuss a few problems which point to the need for
improved stellar evolution models of halo stars. Current stellar evolution
models do not match the observed $^7$Li abundance patterns, suggesting that the
input physics and/or the assumptions used in constructing the models are in
need of revision. It appears that all halo stars have suffered some $^7$Li
depletion, implying that the primordial $^7$Li abundance is higher than that
presently observed in hot halo stars. Observations of abundances of various
elements in globular cluster giant branch stars have suggested for some time
now that some form of deep mixing, which is not present in theoretical models,
occurs in halo stars. The driving mechanism for this mixing, and its
incorporation into stellar models remain one of the key problems in stellar
modeling. Current theoretical isochrones are able to provide a good match to
observed colour-magnitude diagrams. However, there is some evidence that the
theoretical luminosity functions are in disagreement with observations. This is
an area which requires further study, as it suggests that the relative main
sequence/giant branch lifetimes predicted by the models are incorrect. A
discussion of some of the uncertainties involved in determining the ages of
globular clusters is presented. The absolute ages of globular clusters provide
a lower bound to the age of the universe, and so are of great interest to
cosmologists. Unfortunately, present uncertainties in stellar models lead to a
rather large range in the inferred ages of globular clusters of 11 -- 18 Gyr.
