In this chapter we set out issues central to the investigation of computer support for collaborative writing, extending previous work to develop cognitive support systems for single writers (Sharples and O’Malley 1988; Sharples et al. 1989). The most successful computer tools to support complex tasks such as writing are those that fit in with the user’s normal patterns of work (Norman 1986). All writers have strategies of working that suit the context of the task and that have been acquired over many years, through apprenticeship and trial and error. It is difficult to uncover and analyse these strategies, and more difficult still to design computer systems that will support them. Our method for single-person writing was to develop a task model that drew on research in the writing process and to extend it through empirical studies of those aspects of writing (for example, the writer’s use of external representations such as notes and plans) that could be supported by computer tools.