The largest uncertainties in the Standard Model calculation of the anomalous magnetic moment of the muon come from hadronic contributions. In particular, in a few years the sub-leading hadronic light-by-light (HLbL) contribution might dominate the theory uncertainty. We present a dispersive description of the HLbL tensor, which is based on unitarity, analyticity, crossing symmetry, and gauge invariance. This opens up the possibility of a data-driven determination of the HLbL contribution to the muon anomalous magnetic moment with the aim of reducing model dependence and achieving a reliable error estimate.
