  We use rigorous QCD dispersion relations to derive model-independent bounds
on the B -> pi l nu, D -> pi l nu and D -> K 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
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 CKM mixing parameters.
