  We prove $k_T$ factorization theorem in perturbative QCD (PQCD) for exclusive
processes by considering $\pi\gamma^*\to \gamma(\pi)$ and $B\to\gamma(\pi)
l\bar\nu$. The relevant form factors are expressed as the convolution of hard
amplitudes with two-parton meson wave functions in the impact parameter $b$
space, $b$ being conjugate to the parton transverse momenta $k_T$. The point is
that on-shell valence partons carry longitudinal momenta initially, and acquire
$k_T$ through collinear gluon exchanges. The $b$-dependent two-parton wave
functions with an appropriate path for the Wilson links are gauge-invariant.
The hard amplitudes, defined as the difference between the parton-level
diagrams of on-shell external particles and their collinear approximation, are
also gauge-invariant. We compare the predictions for two-body nonleptonic $B$
meson decays derived from $k_T$ factorization (the PQCD approach) and from
collinear factorization (the QCD factorization approach).
