[en] The response of nucleons and complex nuclei to an external electromagnetic probe at intermediate and high energy is illustrated by considering both inclusive and semi-inclusive electron scattering. Gauge invariance and current conservation allow one to define form factors for elastic scattering and structure functions for inelastic scattering which contain the relevant information on the structure and dynamics of the target hadron system under investigation. They are derived and discussed in some specific examples, also taking into account polarization observables. (orig.)