  We conduct a series of comparisons between spectroscopic and photometric
observations of globular clusters and stellar models to examine their
predictive power. Data from medium-to-high resolution spectroscopic surveys of
lithium allow us to investigate first dredge-up and extra mixing in two
clusters well separated in metallicity. Abundances at first dredge-up are
satisfactorily reproduced but there is preliminary evidence to suggest that the
models overestimate the luminosity at which the surface composition first
changes in the lowest-metallicity system. Our models also begin extra mixing at
luminosities that are too high, demonstrating a significant discrepancy with
observations at low metallicity. We model the abundance changes during extra
mixing as a thermohaline process and determine that the usual diffusive form of
this mechanism cannot simultaneously reproduce both the carbon and lithium
observations. Hubble Space Telescope photometry provides turnoff and bump
magnitudes in a large number of globular clusters and offers the opportunity to
better test stellar modelling as function of metallicity. We directly compare
the predicted main-sequence turn-off and bump magnitudes as well as the
distance-independent parameter $\Delta M_V ~^{\rm{MSTO}}_{\rm{bump}}$. We
require 15 Gyr isochrones to match the main-sequence turn-off magnitude in some
clusters and cannot match the bump in low-metallicity systems. Changes to the
distance modulus, metallicity scale and bolometric corrections may impact on
the direct comparisons but $\Delta M_V ~^{\rm{MSTO}}_{\rm{bump}}$, which is
also underestimated from the models, can only be improved through changes to
the input physics. Overshooting at the base of the convective envelope with an
efficiency that is metallicity dependent is required to reproduce the
empirically determined value of $\Delta M_V ~^{\rm{MSTO}}_{\rm{bump}}$.
