Constraints on form factors for exclusive semileptonic heavy to light meson decays.

We use rigorous QCD dispersion relations to derive model-independent bounds on the {ital {bar B}}{r_arrow}{pi}{ital l}{bar {nu}}, {ital D}{r_arrow}{pi}{ital {bar l}}{nu}, and {ital D}{r_arrow}{ital {bar K}{bar l}}{nu} form factors. These bounds are particularly restrictive when the value of the observable form factor at one or more kinematic points is assumed. With reasonable assumptions we find {ital f}{sub {ital B}}{le}195 MeV and that the shape of the form factor becomes severely constrained. These constraints are useful both for model discrimination and for model-insensitive extraction of Cabibbo-Kobayashi-Maskawa mixing parameters.