Report of the working group on radiative corrections at HERA

In inclusive deep-inelastic lepton-nucleon scattering, the radiative 'tails' originating from processes at Q 2 values from the interval Q 2 meas >= Q 2 > 0 contribute to measurements at Q 2 = Q 2 meas and a knowledge of the structure functions in this Q 2 interval is necessary. We review and compare existing parametrizations of structure functions and their extrapolations into the as-yet unexplored very low-Q 2 regions and discuss the resulting uncertainties for leptonic radiative corrections. The unfolding procedures to determine structure functions from observed cross sections by the experiments NMC, ZEUS and H1 are compared with each other.