  The success of the experimental program at the Tevatron re-inforced the idea
that precision physics at hadron colliders is desirable and, indeed, possible.
The Tevatron data strongly suggests that one-loop computations in QCD describe
hard scattering well. Extrapolating this observation to the LHC, we conclude
that knowledge of many short-distance processes at next-to-leading order may be
required to describe the physics of hard scattering. While the field of
one-loop computations is quite mature, parton multiplicities in hard LHC events
are so high that traditional computational techniques become inefficient.
Recently new approaches based on unitarity have been developed for calculating
one-loop scattering amplitudes in quantum field theory. These methods are
especially suitable for the description of multi-particle processes in QCD and
are amenable to numerical implementations. We present a systematic pedagogical
description of both conceptual and technical aspects of the new methods.
