This paper addresses a central issue in secondary school geometry, namely the role of proof. In an integrated mathematics and science curriculum, the role of proof as a process of conjecturing, explaining, and justifying within a small-group setting is analyzed. Particular attention is given to the students' use of empirical evidence and algebraic representations. Each class session of this unit was videotaped, and during small group work, the focus group was videotaped. Written work and computer work done by the group were made available to the researcher for analysis. Extensive field notes were taken by the researcher during class sessions and videotapes of class sessions were reviewed and transcribed for more detailed analysis. (Author/MKR)