This paper is concerned with the development of a language/action perspective in the Cosmos project. We emphasize the importance of seeing cooperative work in terms of participants' communicative actions. In contrast to some explorations of speech act theory, we argue that communicative actions should be seen as essentially embedded in dialogical contexts. In particular, we attempt to show the relevance of concepts derived from the analysis of actually occurring conversations, for computer mediated communication in general and cooperative work in particular. We distinguish between local and global structuring of communication and argue that many group working situations combine both sorts. These observations have influenced our work in the Cosmos project on the design of a structure definition language (SDL) by means of which users can configure their computer mediated communication environment. We describe SDL and show how its interpretation is influenced by our conversation analytic approach. We illustrate our arguments with an example of cooperative document preparation.