When teaching becomes learning

This research examines the role of teachers' content knowledge during the implementation of mathematics education reform. Current mathematics education reform efforts require teachers to learn in the act of teaching. I claim that this learning occurs as teachers negotiate among 3 areas of their content knowledge: their understanding of the subject matter, view of the curriculum materials, and knowledge of student learning. The data for this study come from observations and videotapes of 2 teachers implementing a reform-based linear-functions unit in a high school algebra class. The focus of the article is a detailed analysis of 1 lesson that illustrates the process through which these negotiations occur and the learning that takes place as a result.