  Quark-hadron duality is studied in a systematic way for polarized and
unpolarized structure functions, by taking into account all the available data
in the resonance region. In both cases, a precise perturbative QCD based
analysis of the integrals of the structure functions in the resonance region
has been performed: non perturbative contributions have been disentangled,
higher twist contributions have been evaluated and compared with the ones
extracted in the deep inelastic scattering region. A different behavior for the
unpolarized and polarized structure functions at low Q^2 is found.
