Theoretical Uncertainties in the QCD

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 αs from scaling violations in deep–inelastic scattering are estimated for the kinematic regime accessible at HERA. To appear in the Proceedings of the 1996 Zeuthen Workshop on Elementary Particle Theory: QCD and QED in Higher Orders, Rheinsberg, Germany, April 1996, Nucl. Phys. B (Proc. Suppl.).