Measuring Fn2/Fp2 and d/u:: Past and future

The reliability of the existing extraction of the nucleon structure function and quark distribution ratios F-2(n)/F-2(P) and d/u from proton and deuteron deep inelastic measurements is discussed. Future measurements expected to provide reliable data for the two ratios, free of theoretical uncertainties, are presented. Particular emphasis is given in i) semi-inclusive inelastic electron-deuteron measurements with detection of backward spectator protons and ii) inelastic measurements of electron scattering from the He-3 and H-3 mirror nuclei. These data are expected to test perturbative and non-perturbative mechanisms of spin-flavor symmetry breaking in the nucleon, and constrain the structure function parametrizations needed for the interpretation of high energy electron-proton, proton-proton and proton-antiproton collider data.