  The differences are discussed between various next-to-leading order
prescriptions for the QCD evolution of parton densities and structure
functions. Their quantitative impact is understood to an accuracy of 0.02\%.
The uncertainties due to the freedom to choose the renormalization and
factorization scales are studied. The quantitative consequences of the
different uncertainties on the extraction of the strong coupling constant
$\alpha_s$ from scaling violations in deep--inelastic scattering are estimated
for the kinematic regime accessible at HERA.
