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We use rigorous QCD dispersion relations to derive model-independent bounds on the B→ π l ν, D→ π l ν, and D→ Klν 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 f B≤ 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.