  We argue that a Tau-Charm factory (TCF) could improve our knowledge of CKM
matrix elements since it is an incomparable tool to check the models and
methods applied to extract $V_{bu}$ from $B$ decay partial widths.
  We report on some recent proposals to improve on parton model. Turning to
exclusive decays, we compare the predictions from quark models, QCD sum rules,
effective Lagrangians and lattice QCD. Quark models have anticipated on heavy
quark symmetry. Their difficutly to account for the $q^2$ dependence might be
partly cured by relativistic corrections. QCD sum rules and lattice seem to
disagree on the $q^2$ dependence of axial form factors. We discuss the
extrapolation from $D$ to $B$. Present uncertainties do not allow to predict
reliably the $B\ra\pi,\rho l \nu$ matrix elements. We argue that QCD sum rules
are in a good position to study $q^2$ dependence down to $q^2=0$. Lattice QCD
is able to check the heavy quark scaling laws for the heavy to light
semileptonic decays. It seems to confirm the increase of $A_2/A_1$ from $D$ to
$B$ meson predicted by the scaling laws. Finally semileptonic decays in a TCF
would give incomparable informations on $K\pi$ phase shifts, and on $K^{**}$
resonances.

High Energy Physics - Phenomenology