Because change in mathematics teachers' instructional practices is fundamental to the vision of school mathematics education embodied in the NCTM Standards documents, it is critical that mathematics teacher educators develop a thorough understanding of the process of change in teaching practice. To date, the process of change has remained somewhat intractable. In order to better understand the process, a model for conceptualizing teacher change developed during a two-year study of mathematics teachers' implementation of an innovative curriculum program. The model is based on a constructivist view of teaching and learning and evolved as the study unfolded. Contains 19 references. (Author/MKR)