HERA, the first electron-proton collider, has been delivering luminosity since 1992. It is the natural extension of an impressive series of fixed-target leptonnucleon scattering experiments. The increase of a factor ten 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 sizeable 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, allowed detailed QCD studies and 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.