  In this paper we consider the bilocal field approach for $QCD$. We obtain a
bilocal effective meson action with a potential kernel given in relativistic
covariant form. The corresponding Schwinger--Dyson and Bethe--Salpeter
equations are investigated in detail. By introducing weak interactions into the
theory we study heavy meson properties as decay constants and semileptonic
decay amplitudes. Thereby, the transition from the bilocal field description to
the heavy quark effective theory is discussed. Considering as example the
semileptonic decay of a pseudoscalar $B$--meson into a pseudoscalar $D$--meson
we obtain an integral expression for the corresponding Isgur--Wise function in
terms of meson wave functions.
