The overarching theme of this book can be simply stated: Building on a foundation of biologically based abilities, children construct number via sensorimotor and mental activity. In this chapter, we return to this theme, and we connect it to three additional themes that emerge across chapters: comparing competing models for conceptual change; consideration of multiple concepts for natural numbers, fractions, and integers; and understanding interrelations of conceptual and procedural knowledge in the construction of number. We close by suggesting ways that psychologists and mathematics educators might move forward with interdisciplinary research that addresses important questions about the construction of number. Indeed, the chapters in this volume chart many possible paths. [For the complete volume, "Constructing Number: Merging Perspectives from Psychology and Mathematics Education. Research in Mathematics Education," see ED616587.]