  We consider direct diphoton production in hadron collisions. We compute the
next-to-next-to-leading order (NNLO) QCD radiative corrections at the
fully-differential level. Our calculation is based on the q_T subtraction
formalism and it is implemented in a parton level Monte Carlo program, which
allows the user to apply arbitrary kinematical cuts on the final-state photons
and the associated jet activity, and to compute the corresponding distributions
in the form of bin histograms. We present selected numerical results related to
Higgs boson searches and diphoton studies performed at the LHC and the
Tevatron, and we show how the NNLO corrections to diphoton production are
relevant to understand the main background of the decay channel (H -> gamma
gamma) of the Higgs boson H.
