  Omnes dispersion relations make a connection between form factors for
exclusive semileptonic decays and phase shifts in the corresponding elastic
scattering channels. We describe two applications. In the first, we use lattice
form factor calculations to learn about phase shifts in elastic s-wave
isospin-1/2 channels for K pi, B pi, D pi and D K scattering. The aim of the
second application is to make the determination of the CKM matrix element
magnitude |Vub| from exclusive semileptonic B to pi decays competitive with
that from inclusive decays. Here we use many subtractions in an Omnes
dispersion relation to motivate a simple fitting function, allowing data to
constrain the q^2 shape of the differential decay rate and theory, primarily
lattice results, to provide normalisation via form factor values.
