  We summarize recent studies of realistic nonsupersymmetric Grand Unified
Theories (GUTs) extended with a global $U(1)_{\rm PQ}$ symmetry, so as to
accommodate the axion solution to the strong CP problem. Aside from solving the
CP problem and unifying the gauge structure of the SM, these models can also
yield realistic spectra and mixings, including neutrino masses, and allowing
for a consistent cosmological history that accounts for inflation, dark matter
and baryogenesis. In our studies of $SO(10)$ and $SU(5)$ theories, we
determined the mass and couplings of the axion in terms of the relevant
threshold scales, and assessed how the former are constrained from the
requirements of gauge coupling unification, proton decay searches and collider
bounds. The axion mass ends up being rather constrained for GUT scale axions,
particularly in the case of $SU(5)$, and could be probed by upcoming dark
matter experiments, such as ABRACADABRA and CASPEr-Electric.
