This book provides examples of the ways in which 9-12 grade mathematics teachers from across North America are engaging in research. It offers a glimpse of the questions that capture the attention of teachers, the methodologies that they use to gather data, and the ways in which they make sense of what they find. The focus of these teachers' investigations into mathematics classrooms ranges from students' understanding of content to pedagogical changes to social issues. Underlying the chapters is the common goal of enabling students to develop a deep understanding of the mathematics they learn in their classrooms. By opening their analysis of their classroom practice to our inspection, these courageous teachers have invited us to think along with them and to learn more about our own teaching as a result. By sharing their work, they have given the mathematics education community an important opportunity. Everyone who reads this book-teachers, researchers, teacher-researchers, policy makers, administrators, and others interested in mathematics education-can learn from the findings and the light that they shed on issues important to mathematics education. This book, and the series of which it is a part, also provides the opportunity to step back and reflect on what can be learned about research from teachers who have engaged in the process. Areas of insight include: (a) the importance of collaboration and participation in communities that value research, (b) the potential of teacher research as a way to warrant teacher practice, (c) the power of video and other artifacts of teaching to support classroom inquiry, (d) connections between teaching and research, and (e) the publication process as professional development.