  The largest uncertainty in calculations of hydrogen ground-state hyperfine
splitting comes from corrections due to proton stucture. We review these
corrections, with special mention of the inelastic, or polarizability,
corrections which have been recently re-evaluated. Summing up the arguably best
current values for the calculated corrections leaves us 1-2 ppm short of the
experimental data. We speculate how one may explain this shortfall, and along
the way broadly outline the derivations of the relevant formulas, attempting to
explain how certain terms come to appear and commenting on the use of
unsubtracted dispersion relations.
