For many years, experiments studying the scattering of electrons, muons, and neutrinos on nucleons have provided abundant information on the structure of the nucleon and on the properties of the basic forces acting between elementary constituents. With the opening of the electron-proton collider HERA (Hadron-Electron-Ring Accelerator) at DESY (Deutsches Elektronen-Synchrotron) in Hamburg, Germany, the available center-of-mass energy in the lepton-nucleon system increased by more than an order of magnitude, corresponding to a tenfold improvement in spatial resolution compared to that provided in fixed-target experiments.