  Non-supersymmetric Grand Unified $SO(10)\times U(1)_{\rm PQ}$ models have all
the ingredients to solve several fundamental problems of particle physics and
cosmology -- neutrino masses and mixing, baryogenesis, the non-observation of
strong CP violation, dark matter, inflation -- in one stroke. The axion - the
pseudo Nambu-Goldstone boson arising from the spontaneous breaking of the
$U(1)_{\rm PQ}$ Peccei-Quinn symmetry - is the prime dark matter candidate in
this setup. We determine the axion mass and the low energy couplings of the
axion to the Standard Model particles, in terms of the relevant gauge symmetry
breaking scales. We work out the constraints imposed on the latter by gauge
coupling unification. We discuss the cosmological and phenomenological
implications.
