Demand for more advanced Web applications is the driving force behind Web browser evolution. Recent requirements for Rich Internet Applications, such as mashing-up data and background processing, are emphasizing the need for building and executing Web applications as a coordination of browser execution contexts. Since development of such Web applications depends on cross-context communication, many browser primitives and client-side frameworks have been developed to support this communication. In this paper we present a systematization of cross-context communication systems for Web browsers. Based on an analysis of previous research, requirements for modern Web applications and existing systems, we extract a framework for classifying cross-context communica-tion systems. Using the framework, we evaluate the current ecosystem of cross-context communication and outline directions for future Web research and engineering.