HERA collider physics

HERA, the first electron-proton collider, has been delivering luminosity since 1992. It is the natural extension of an impressive series of fixed-target lepton-nucleon scattering experiments. The increase of a factor of 10 in center-of-mass energy over that available for fixed-target experiments has allowed the discovery of several important results, such as the large number of slow partons in the proton and the sizable diffractive cross section at large $Q^2.$ Recent data point to a possible deviation from standard-model expectations at very high $Q^2,$ highlighting the physics potential of HERA for new effects. The HERA program is currently in a transition period. The first six years of data taking have primarily elucidated the structure of the proton, have allowed detailed QCD studies, and have had a strong impact on the understanding of QCD dynamics. The coming years will bring the era of electroweak studies and high $Q^2$ measurements. This is therefore an appropriate juncture at which to review HERA results.