This paper explores the use of data for evidence in the task of building dialogue in an action research project developed for student teachers in the University of Wisconsin "Action Research on Action Research" project. Three particular uses are found. First, data in the form of information is gathered on a topic under discussion or investigation, namely, the Action Research Project. Each student as well as the supervisor has data on her/his own focus which can be shared with the group. Dealing with this data becomes the focus of a second phase of data gathering: data about how the group processes are working in meeting goals, whether jointly set as co-researchers or in relation to individually specific situations. Finally, there is the data about what is being learned about action research, which is the focus of the supervisors in their joint project. The point of collecting the data is to further the communicative action of members of the group and to promote greater articulation and understanding of the specific interests and tasks of student teaching. Brief case histories of students, focusing on classroom management and discipline as their projects, are used to demonstrate how the discussion of data assisted group members to build and articulate their own theories. (JD)