The hadronic light-by-light contribution to the muon's anomalous magnetic moment

In view of the current 3 - 4 $\sigma$ deviation between theoretical and experimental values for the muon’s anomalous magnetic moment, we review the ongoing efforts in constraining the hadronic light-by-light contribution to $a_{\mu }$ by using dispersive techniques combined with a dedicated experimental program to obtain the required hadronic input.