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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 thedispersion approach. Employing the method of regions, factorization of the $B\to \gamma^{\ast}$ form factors is demonstrated explicitly, at one loop, for aspace-like hard-collinear photon. The two-particle soft (end-point)contribution is shown to be suppressed by one power of $\Lambda/m_b$, in theheavy quark limit, compared with the leading power contribution computed fromQCD factorization. I further report the recent calculation on thethree-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 thetwo-particle counterparts, as already speculated from the rapidity divergencein the corresponding factorization formulae. Phenomenological implications ofthe subleading power contributions to the $B \to \gamma \ell \nu$ amplitude arealso addressed in detail, focusing on the determination of the inverse momentof the leading-twist $B$-meson distribution amplitude.

Subleading power corrections in radiative leptonic $B$ decay