  We prove factorization theorem for the processes $\pi\gamma^*\to\gamma$ and
$\pi\gamma^*\to\pi$ to leading twist in the covariant gauge by means of the
Ward identity. Soft divergences cancel and collinear divergences are grouped
into a pion wave function defined by a nonlocal matrix element. The gauge
invariance and universality of the pion wave function are confirmed. The proof
is then extended to the exclusive $B$ meson decays $B\to\gamma l\bar\nu$ and
$B\to\pi l\bar\nu$ in the heavy quark limit. It is shown that a light-cone $B$
meson wave function, though absorbing soft dynamics, can be defined in an
appropriate frame. Factorization of the $B\to\pi l\bar\nu$ decay in $k_T$
space, $k_T$ being parton transverse momenta, is briefly discussed. We comment
on the extraction of the leading-twist pion wave function from experimental
data.
