The hadronic vacuum polarization, Fig. 3.1, is the largest hadronic effect on the muon anomalous magnetic moment; it has to be known with high precision to match the existing experimental effort. Given the low value of the muon mass, this task seems formidable because at low energies hadronic interactions are strong. We are rescued by a dispersion representation of the photon propagator that relates hadronic vacuum polarization contribution to a μ and the experimentally measured e + e - annihilation cross section into hadrons. In principle, this allows to account for the effects of strong interactions exactly. Such an approach puts, however, a significant burden on the experiment, requiring high precision measurement of the e + e - annihilation cross section.