Context-aware middleware encompasses uniform abstractions and reliable services for common operations, supports for most of the tasks involved in dealing with context, and thus simplifying the development of context-aware applications. In this paper1, we address some key issues of a middleware for context-aware ubiquitous computing, ranging from design considerations of a unified sensing framework, formal modeling and representation of the real world, pluggable reasoning engines for high-level contexts, and context delivery-runtime service composition mechanisms. Our implementation experience indicates that a comprehensive approach throughout the system layers results in a flexible and reusable middleware architecture.