We summarize recent studies of realistic nonsupersymmetric Grand Unified Theories (GUTs) extended with a global U(1)_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.