This longitudinal study documents the development of mathematics teachers as they make the transition from preservice to experienced teachers. Collaborative learning environments were created for these teachers in their field-based classrooms during their last year in the teacher-education program. The authors anticipated that these learning environments would prepare more effective novice teachers. A comparison of baseline data with end-of-project data indicated that the participants' beliefs about the teaching and learning of mathematics, as well as their epistemological views, had changed; however, many participants did not base their instructional decisions on a careful analysis of their students' thinking processes. A follow-up study revealed a more accurate picture of the change in participants' beliefs and practices, as some of them, now experienced teachers, exceeded project expectations and had become effective teachers. Factors that either hindered or contributed to the development of teachers' practice are also identified. (Contains 2 tables and 2 figures.)