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Our analyses of the activities performed by users of computer systems show complex patterns of interleaved activities. Current human - computer interfaces provide little support for the kinds of problems users encounter when attempting to accomplish several different tasks in a single session. In this paper we develop a framework for discussing the characteristics of activities, in terms of activity structures, and provide a number of conceptual guidelines for developing an interface which supports activity coordination. The concept of a workspace is introduced as a unifying construct for reducing the mental workload when switching tasks, and for supporting contextually-driven interpretations of the users' activity structures.