  We study the form factors of heavy--to--heavy and heavy--to--light weak
decays using the light--front relativistic quark model. For the
heavy--to--heavy $B \ra D^{(\ast)}$ semileptonic decays we calculate the
corresponding Isgur--Wise function for the whole kinematic region. For the
heavy--to--light $B\ra P$ and $B\ra V$ semileptonic decays we calculate the
form factors at $q^2 = 0$; in particular, we have derived the dependence of the
form factors on the $b$--quark mass in the $m_b \ra \infty$ limit. This
dependence can not be produced by extrapolating the scaling behavior of the
form factors at $q^2_{max}$ using the single--pole assumption. This shows that
the $q^2$ dependence of the form factors in regions far away from the
zero--recoil could be much more complicated than that predicted by the
single--pole assumption.
