The purpose of this article is to provide evidence that teachers’ observations of students’ mathematical activity in research project on students’ development of mathematical ideas can provide rich opportunities for teachers to learn about students’ mathematical reasoning. Nine mathematics teachers and 24 sixth-grade students participated in the IML project, which took place in a middle school, located in an urban, low-income, and minority community in the United States. The results of this study suggest that teachers’ observations of students’ mathematical activity in IML-type settings might help teachers develop an understanding of mathematics that is effective for teaching.