Through the chapters in this volume we learn about the research foci and/ or questions that these classroom teachers are interested in examining, the mathematics content through which they engaged their students in these explorations, the data sources they used to make sense of their focus and questions, and their roles in the research. An overarching theme through all the chapters in this volume is the learning and professional development that occurs through teacher research. What these authors learned about student learning and their own teaching practice far exceeded the focus of their particular research questions. For some, the research validated their beliefs and instructional practices; for others, it deepened or extended their understanding of mathematics, or raised their expectations of students' capabilities. For all, it is fair to say, their research increased their awareness of how students come to know and understand mathematics, and enabled them to gain insight into the complexity of teaching. Lampert (2001) noted, "One reason teaching is a complex practice is that many of the problems a teacher must address to get students to learn occur simultaneously, not one after another" (p. 2). We, as readers, gain a window into these teachers' research within and about the complexity of classroom teaching.