Theoretical aspects of HERA physics

I discuss the theoretical underpinnings for the extraordinary success of perturbative QCD in the description of HERA data. In particular, I examine recent progress in the understanding of perturbative QCD at small x. I explain the relation between evolution equations in Q2 and x, and how they can be used for simultaneous resummation of the relevant large logs at HERA. I 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 unaccessible behavior of QCD in the Regge limit