Among all hadronic contributions to the muon anomalous magnetic moment, the hadronic light-by-light scattering is the one which is most difficult to analyze. This is the consequence of two facts. First, the hadronic light-bylight scattering is largely determined by non-perturbative QCD. Second, it is difficult to relate the hadronic light-by-light scattering contribution to the muon magnetic anomaly to experimental data. Because of these problems, we have to rely on theoretical considerations to estimate the hadronic lightby- light scattering contribution to the muon anomalous magnetic moment.