Abstract: Axions, grand unification, and string theory are each compelling extensions of the Standard Model. I will show that combining these frameworks imposes strong constraints on the QCD axion mass. Using unitarity arguments and explicit string compactifications - such as those from the Kreuzer-Skarke (KS) type IIB ensemble - I will discuss why the axion mass is favored to lie within the range 1e-11 eV < m_a < 1e-8 eV.