HERMES is an experiment under preparation by an international collaboration of physicists from the United States, Germany, Canada, Italy and the Soviet Union to measure the deep inelastic spin‐dependent structure functions of the nucleon at DESY, Hamburg, Germany. The structure functions of the proton and neutron will be determined by a completely new technique using polarized internal gas targets of hydrogen, deuterium, and3He and the longitudinally polarized 35 GeV electron beam of the HERA electron storage ring. Measurement of the deep inelastic spin‐structure of both isospin states of the nucleon at the same kinematics and using the same apparatus will provide a high precision test of the Bjorken sum rule.