Context-aware applications are seen as one of the "killer" application categories of the future, due to their ability to offer personalized services by adapting their behavior according to the users' needs and changing situation. Context-aware applications rely in their operation on a complex set of functionalities (i.e. context-awareness substrates). In order to facilitate the development of novel context-aware applications and achieve efficiency in terms of resource utilization, there is a need for a unified, openly-accessible, scalable context management platform that enables the dynamic discovery, composition, and reuse of context-awareness substrates by various context-aware applications. The lack of such platform is a major impediment to the fast and resource efficient development of context aware applications. In this paper, we propose a novel virtualized context management platform in the cloud, in which a shared pool of virtualized context-awareness substrates can be offered by different providers, and leased on demand. Those substrates can be dynamically discovered and composed to enable fast and cost-effective development of a variety of context-aware applications. The proposed platform relies on a new business model which introduces the sensors substrate provider and the broker as new roles in the traditional cloud business model. A detailed software architecture and preliminary prototype implementation are also presented.I.