Theoretical Approaches to $B\to K^{\ast }\gamma$

We describe two recent theoretical calculations of the $B\to K^{\ast }\gamma$ decay. The first method is based on an effective lagrangian incorporating chiral symmetry for the light degrees of freedom and the heavy quark spin-flavor symmetries for the heavy quarks. Using these symmetries, together with the hypothesis of polar q${}^2$ dependence of the semileptonic form factors, one is able to relate $B\to K^{\ast }\gamma$ and $D\to K^{\ast }\mathrm{\ell }\nu$ decays with the result $ R , =, \Gamma(B \to K^* \gamma) / \Gamma(b \to s \gamma) = 0.09 \pm 0.03$. The second method is based on 3-point function QCD sum rules and gives the result $R=0.17\pm 0.05$. Both results are in reasonable agreement with recent data from the CLEO II Collaboration.