Theory of Semi-- and Nonleptonic Decays of Heavy Mesons

I review some of the recent developments in the theoretical description of weak inclusive decays of heavy mesons. The topics cover the value of $\mathrm{\mid }{V}_{cb}\mathrm{\mid }$ as extracted from semileptonic inclusive decays and a short discussion of the theoretical errors. I also present the results of a recent calculation of next–to–leading order corrections to nonleptonic inclusive B decays which allows an improved prediction of the semileptonic branching ratio of B mesons.