The confluence of ubiquitous computing, anywhere/anytime access to information resources, and scalable computing enables the construction of Active Spaces. In such a Space, a spectrum of computation and communication devices seamlessly augment human thought and activity with digital information, processing, and analysis to provide an observed or imagined world that is automated and enhanced by ... [Show full abstract] the behavioral context of its users. Large numbers of inexpensive computing devices provide new functionality, enhance user productivity and ease everyday tasks. In home, office and public spaces, ubiquitous computers will unobtrusively augment work or recreational activities with information technology that optimizes the environment for people's needs. The power of such a computer infrastructure has three contributing factors: the translating of information to and from physical properties, the computers and their ability to transform data, and the cooperative computational environment that results from embedding these devices in a network. Given the impact of the Internet, this last factor, the computational capabilities of an Active Space, is the likely long-term benefit of the current information technology revolution. However, the benefit cannot be achieved without devising a new form of operating system that enables applications to be built and run in Active Spaces. The operating system must manage the resources of a physical space and its devices for a user and his applications. This paper addresses the fundamental issues in the design of such a system.