  We discuss the theoretical underpinnings for the extraordinary success of
perturbative QCD in the description of HERA data. In particular, we examine
recent progress in the understanding of perturbative QCD at small x. We explain
the relation between evolution equations in Q^2 and x, and how they can be used
for simultaneous resummation of the relevant large logs at HERA. We show that
while the HERA data can be understood within our current knowledge of the
perturbative expansion of QCD, they pose stringent constraints on the
perturbatively inaccessible behaviour of QCD in the Regge limit.
