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. [S0034-6861(99)00305-0].