The Hadron-Electron Ring Accelerator facility (HERA) consists of two storage rings. In one of them, protons circulate counterclockwise with an energy of 900 GeV, in the other ring electrons or positrons circulate clockwise with an energy of 27.5 GeV. The two rings are brought together in two points to produce collisions. On each of these collision points detectors are placed to study the interaction of electron and protons. HERA offers the possibility of carrying out experiments with polarized beams. The degree of polarization of electrons and/or positrons of up to 60&percent; is very stable. Since 1995 the collaboration HERMES is running an experiment designed to provide information on the spin of the nucleon. A fourth experiment makes use of the proton beam halo to produce B mesons. HERA-B intend to investigate the violation of CP symmetry in the decay of B mesons. Here we try to illustrate concepts of particle physics and the experimental methods used at HERA and do not intend to give an exhaustive summary of the physics program. ©2000 American Institute of Physics.