Context-aware computing is a central theme of ubiquitous computing. In order to facilitate the development and proliferation of context-aware applications in ubiquitous environment, a context-aware middleware is indispensable. Although many middleware systems have been presented in recent years, context-aware applications are still not very widely deployed in our real-life settings. Several key issues are not properly addressed, such as low cost (light-weight), QoS, spontaneous access, prototype verification and testing, balancing autonomy and user control, safety and privacy, scalability, etc. Context-aware middleware and applications require further scientific research for extending the current capabilities to be more effective and applicable in the real world.