  We investigate the effect of the structure function F_L in global parton
analyses of deep inelastic and related hard scattering data. We perform NLO and
NNLO analyses which include the reduced cross section HERA data at high y, as
well as earlier direct measurements of F_L. We find that the NNLO analysis
gives a better description of F_L at low x than that performed at NLO.
Nevertheless the data show evidence of the need of further contributions to
F_L, which may be of higher-twist origin. We study such corrections both
phenomenologically and theoretically via a renormalon approach. The
higher-twist corrections extracted from a successful fit to the data are in
general agreement with the theoretical expectations, but there is still room
for alternative theoretical contributions, particularly at low x and Q^2. The
importance of future measurements of F_L is emphasized.
