  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 $q^2=0$. This allows us
to evaluate the form factors at the same value of $q^2$, 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
$A_{0}(0)$. 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 $\rho$ 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 $q^2$ 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
$B \rightarrow K^{*} \ell \bar{\ell}$ at $q^2=0$ 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 $B \rightarrow K^{*}\nu \bar{\nu}$ as a
possibility. As the mass of the decaying particle increases we note that the
$SU(3)$ symmetry becomes badly broken at $q^2=0$.
