We review the present status of theoretical attempts to calculate the
semileptonic charm and bottom decays and then present a calculation of these
decays in the light--front frame at the kinematic point q2=0. This allows us
to evaluate the form factors at the same value of q2, even though the
allowed kinematic ranges for charm and bottom decays are very different. Also,
at this kinematic point the decay is given in terms of only one form factor
A0(0). For the ratio of the decay rates given by the E653 collaboration we
show that the determination of the ratio of the Cabibbo--Kobayashi--Maskawa
(CKM) matrix elements is consistent with that obtained from the unitarity
constraint. At present, though, the unitarity method still has greater
accuracy. Since comparisons of the semileptonic decays into ρ and either
electrons or muons will be available soon from the E791 Fermilab experiment, we
also look at the massive muon case. We show that for a range of q2 the
SU(3)F symmetry breaking is small even though the contributions of the
various helicity amplitudes becomes more complicated. For B decays, the decay
B→K∗ℓℓˉ at q2=0 involves an extra form factor
coming from the photon contribution and so is not amenable to the same kind of
analysis, leaving only the decay B→K∗ννˉ as a
possibility. As the mass of the decaying particle increases we note that the
SU(3) symmetry becomes badly broken at q2=0.Comment: Latex, 19 pages, two figures are attached, a minor change in the
manuscript related to thi