  Based on dispersion theory, we present a formalism for a model-independent
evaluation of the hadronic light-by-light contribution to the anomalous
magnetic moment of the muon. In particular, we comment on the definition of the
pion pole in this framework and provide a master formula that relates the
effect from pi pi intermediate states to the partial waves for the process
gamma^* gamma^* --> pi pi. All contributions are expressed in terms of on-shell
form factors and scattering amplitudes, and as such amenable to an experimental
determination.
