I investigate what a direct measurement of the longitudinal structure function
FL(x,Q2) could teach us about the structure of the proton and the best way in
which to use perturbative QCD for structure functions. I assume HERA running
at a lowered beam energy for approximately 4-5 months and examine how well
the measurement could distinguish between different theoretical approaches. I
conclude that such a measurement would provide useful information on how to
calculate structure functions and parton distributions at small x
