Recent Developments in the Theory of Heavy-Quark Decays

I report on recent developments in the heavy-quark effective theory and its application to $B$ meson decays. The parameters of the effective theory, the spin-flavor symmetry limit, and the leading symmetry-breaking corrections to it are discussed. The results of a QCD sum rule analysis of the universal Isgur-Wise functions that appear at leading and subleading order in the $1/m_Q$ expansion are presented. I illustrate the phenomenological applications of this formalism by focusing on two specific examples: the determination of $V_{cb}$ from the endpoint spectrum in semileptonic decays, and the study of spin-symmetry violating effects in ratios of form factors. I also briefly comment on nonleptonic decays.