  We compute long distance effects on the photon energy spectrum in inclusive
radiative decays of $B$ mesons using light-cone expansion and heavy quark
effective theory. We show that for sufficiently high photon energy the leading
nonperturbative QCD contribution is attributed to the distribution function.
The distribution function is found to be universal in the sense that the same
distribution function also encodes the leading nonperturbative contribution to
inclusive semileptonic decays of $B$ mesons at large momentum transfer. Some
basic properties of the distribution function are deduced in QCD. Ways of
extracting the distribution function directly from experiment and their
implications are discussed. The theoretically clean methods for the
determination of $|V_{ts}|$ are described.
