There is a documented need for more opportunities for teachers to learn about students' mathematical reasoning. This article reports on the experiences of a group of elementary and middle school mathematics teachers who participated as interns in an after-school, classroom-based research project on the development of mathematical ideas involving middle-grade students from an urban, low-income, minority community in the United States. For 1 year, the teachers observed the students working on well-defined mathematical investigations that provided a context for the students' formation of particular mathematical ideas and different forms of reasoning in several mathematical content strands. The article describes insights into students' mathematical reasoning that the teachers were able to gain from their observations of the students' mathematical activity. The purpose is to show that teachers' observations of students' mathematical activity in research sessions on students' development of mathematical ideas can provide opportunities for teachers to learn about students' mathematical reasoning.