This article describes how an object-oriented approach can be applied to the architectural design of a spoken language dialog system with the aim of facilitating the modification, extension, and reuse of discourse-related expertise. The architecture of the developed system is described and a functionally similar VoiceXML system is used to provide a comparative baseline across a range of modification and reuse scenarios. It is shown that the use of an object-oriented dialog manager can provide a capable means of reusing existing discourse expertise in a manner that limits the degree of structural decay associated with system change.