Semantic Web technologies offer the prospect of significantly reducing the amount of effort required to integrate existing enterprise functionality in support of new composite processes.-- whether within a given organization or across multiple ones. A significant body of work in this area has aimed to fully automate this process, while assuming that all functionality has already been encapsulated in the form of semantic web services with rich and accurate annotations. In this article, we argue that this assumption is often unrealistic. Instead, we describe a mixed initiative framework for semantic web service discovery and composition that aims at flexibly interleaving human decision making and automated functionality in environments where annotations may be incomplete and even inconsistent. An initial version of this framework has been implemented in SAP's Guided Procedures, a key element of SAP's Enterperise Service Architecture (ESA).