Soft and Hard Pomerons

Regge theory provides an excellent description of small-x structure-function data from Q 2 = 0 up to the highest available values. The large-Q 2data should also be described by perturbative QCD: the two descriptions must agree in the region where they overlap. However, at present there is a serious lack in our understanding of how to apply perturbative QCD at small x. The usual lowest-order or next-to-lowest order expansion is not valid, at least not until Q 2becomes much larger than is usually assumed; a resummation is necessary, but as yet we do not know how to do this resummation.