  We derive the hard functions for all 2->2 processes in massless QCD up to
next-to-next-to-leading order (NNLO) in the strong coupling constant. By
employing the known one- and two-loop helicity amplitudes for these processes,
we obtain analytic expressions for the ultraviolet and infrared finite,
minimally subtracted hard functions, which are matrices in color space. These
hard functions will be useful in carrying out higher-order resummations in
processes such as dijet and highly energetic top-quark pair production by means
of soft-collinear effective theory methods.
