  We review recent developments in the calculation of QCD loop amplitudes with
several external legs, and their application to next-to-leading order jet
production cross-sections. When a number of calculational tools are combined
together --- helicity, color and supersymmetry decompositions, plus unitarity
and factorization properties --- it becomes possible to compute multi-parton
one-loop QCD amplitudes without ever evaluating analytically standard one-loop
Feynman diagrams. One-loop helicity amplitudes are now available for processes
with five external partons (ggggg, q\bar{q}ggg and q\bar{q}q'\bar{q}'g), and
for an intermediate vector boson V \equiv \gamma^*,Z,W plus four external
partons (Vq\bar{q}gg and Vq\bar{q}q'\bar{q}'). Using these amplitudes,
numerical programs have been constructed for the next-to-leading order
corrections to the processes p\bar{p} to 3 jets (ignoring quark contributions
so far) and e^+e^- to 4 jets.
