  We analyze the double radiative B-meson decays B_s -> gamma gamma and B_d ->
gamma gamma in QCD factorization based on the heavy-quark limit m_b >>
Lambda_QCD. We systematically discuss the various contributions to these
exclusive processes. The dominant effect arises from the magnetic-moment type
transition b -> s(d) gamma where an additional photon is emitted from the light
quark (one-particle reducible diagram). The contributions from one-particle
irreducible diagrams are power suppressed. We argue that they are still
calculable within QCD factorization. They are used to compute the
CP-asymmetries in B -> gamma gamma and to estimate so-called long-distance
contributions in B and D -> gamma gamma. Numerical results are presented for
branching ratios and CP asymmetries.
