[en] Inclusive neutron and Δ⁺⁺ production has been studied extensively in low energy hadron-hadron collisions. Experiments show that the cross section for small momentum transfers is dominated by one pion exchange (OPE). By examining the system recoiling from the produced neutron or Δ⁺⁺ virtual pion-nucleon interactions can be investigated. In low Q² interactions at HERA the production of a leading neutron or Δ⁺⁺ is also dominated by OPE and virtual γπ collisions can be studied. The total γπ cross section can be measured as well as the photoproduction of jets from pions. If OPE is also an important component of deep inelastic scattering, virtual eπ collisions can be investigated, and structure functions of the pion extracted in both neutral and charged current interactions. A forward neutron calorimeter and a leading proton spectrometer detect leading baryons, in effect supplying the experiments with a tagged pion beam. In this paper we review experimental and theoretical aspects of one pion exchange. We estimate cross sections for virtual pion scattering at HERA. We examine some expected characteristics of the final state of the virtual eπ events. (orig.)