Emerging pervasive computing technologies transform the way we live and work by embedding computation in our surrounding environment. To avoid increasing complexity, and allow the user to concentrate on her tasks, applications must automatically adapt to their changing context, the physical and computational environment in which they run. To support these “context-aware” applications we propose a graph-based abstraction for collecting, aggregating, and disseminating context information. The abstraction models context information as events, which are produced by sources, flow through a directed acyclic graph of event-processing operators, and are delivered to subscribing applications. Applications describe their desired event stream as a tree of operators that aggregate low-level context information published by existing sources into the high-level context information needed by the application. The operator graph is thus the dynamic combination of all applications’ subscription trees. In this paper, we motivate our graph abstraction by discussing several applications under development, sketch the architecture of our system (“Solar”) that implements our abstraction, report some early experimental results from the prototype, and outline issues for future research.