First, we will discuss electromagnetic reactions of electrons and hadrons, that is, the processes that can be described by the exchange of photons. This is primarily the electron-positron annihilation into hadrons at center-of-mass energies well below the mass of the Z boson. Equally important is the (inelastic) electron-proton scattering at values of \(\sqrt {\left| {q^{2} } \right|}\) that are small compared to the mass of the W and Z bosons. In the quark model, these reactions are interpreted as electromagnetic scattering processes between electrons and quarks. Once quarks are involved, the electromagnetic interaction can no longer be treated without considering quantum chromodynamics. QCD comes into play in two ways: firstly, through perturbatively calculable corrections to the electromagnetic cross sections, and secondly, through the description of the hadronization of quarks and gluons; the latter is not perturbatively calculable.