We present an architecture to facilitate automated discovery, selection, and composition of semantically-described heterogeneous services using Semantic Web technologies. Our framework has three main features which distinguish it from other work in this area. First, we propose a dynamic, adaptive, and highly fault-tolerant service discovery and composition algorithm. Second, we distinguish between different levels of granularity of loosely coupled workflows. Finally, our framework allows the user to specify and refine a high-level objective.