Semileptonic B to rho and B to π Decays: Lattice and Dispersive Constraints

I present two recent pieces of work on semileptonic $B \to \pi, \rho$ decays where it is shown how lattice QCD calculations can be used to test heavy-quark symmetry and determine phenomenologically relevant quantities despite the limits on these calculations' kinematical reach. The study of semileptonic $B \to \rho$ decays was performed with the UKQCD Collaboration.