  We report on the extraction of the higher twist contributions to F_2 and R =
sigma_L/sigma_T from the global NLO and NNLO QCD fits to lepton nucleon
scattering data over a wide range of Q^2. The NLO fits require both target mass
and higher twist contributions at low Q^2. However, in the NNLO analysis, the
data are described by the NNLO QCD predictions (with target mass corrections)
without the need for any significant contributions from higher twist effects.
An estimate of the difference between NLO and NNLO parton distribution
functions is obtained.
