  I discuss the two-particle subleading power corrections in radiative leptonic
$B \to \gamma \ell \nu$ decay at next-to-leading order in $\alpha_s$ with the
dispersion approach. Employing the method of regions, factorization of the $B
\to \gamma^{\ast}$ form factors is demonstrated explicitly, at one loop, for a
space-like hard-collinear photon. The two-particle soft (end-point)
contribution is shown to be suppressed by one power of $\Lambda/m_b$, in the
heavy quark limit, compared with the leading power contribution computed from
QCD factorization. I further report the recent calculation on the
three-particle subleading power contribution to the on-shell $B \to \gamma$
form factors at tree level and demonstrate that the "soft" and the "hard"
three-particle corrections are of the same power, in contrast to the
two-particle counterparts, as already speculated from the rapidity divergence
in the corresponding factorization formulae. Phenomenological implications of
the subleading power contributions to the $B \to \gamma \ell \nu$ amplitude are
also addressed in detail, focusing on the determination of the inverse moment
of the leading-twist $B$-meson distribution amplitude.
