Nonperturbative Contributions to the Inclusive Rare Decays $B\to X_s\gamma$ and $B\to X_s\ell^+\ell^-$

We discuss nonperturbative contributions to the inclusive rare $B$ decays $B\to X_s\gamma$ and $B\to X_s\ell^+\ell^-$. We employ an operator product expansion and the heavy quark effective theory to compute the leading corrections to the decay rate found in the free quark decay model, which is exact in the limit $m_b\to\infty$. These corrections are of relative order $1/m_b^2$, and may be parameterised in terms of two low-energy parameters. We also discuss the corrections to other observables, such as the average photon energy in $B\to X_s\gamma$ and the lepton invariant mass spectrum in $B\to X_s\ell^+\ell^-$.