Descriptions of human-machine systems should reflect the user's perspective to be helpful for understanding her/his work-environment and for building up mental models needed for human-machine interaction (HMI). A conceptual framework for HMI is presented. It characterizes computer-supported work on three levels: on the first two levels in terms of co-operating roles played by human role players and in terms of functions which may be performed by persons or machines as actors. The dynamics are treated on a third level, called the action level.