  We analyse the Lorentz structures of weak decay matrix elements bewteen meson
states of arbitrary spin. Simplifications arise in the transition amplitudes
for a heavy meson decaying into a light one via a Bethe-Salpeter approach which
incorporates heavy quark symmetry. Phenomenological consequences on several
semileptonic, nonleptonic and FCNC induced decays of heavy flavoured mesons are
derived and discussed.
