Parton densities and structure functions beyond the next-to-leading order

We discuss recent results on the evolution of unpolarized parton densities and structure functions in massless perturbative QCD. Present partial results on the next-to-next-to-leading order (NNLO) evolution kernels prove sufficient for reliable calculations at not too small values of the Bjorken variable, χ > 10−3. One order more can be taken into account at χ ≥ 0.2. Inclusion of these terms considerably reduces the main theoretical uncertainties of determinations of αs (to about 1% at the Z-mass) and the parton densities from structure functions.