Introduction:As the International Group for the Psychology of Mathematics Education (IG PME) grew up, so did constructivism. Reflecting over the role of constructivism in the history of mathematics education is a daunting task, but one which provides an opportunity to reflect on what has been accomplished, honor the contributions of scholars around the world, and identify what remains unfinished or unexplained. In undertaking this task, we divide our treatment into five major sections: (1) The historical precedents of constructivism during the first ten years (1976–85); (2) The debates surrounding the ascendancy of constructivism during the next ten years (1986–95); (3) Our own articulation of key principles of constructivism; (4) Thematic developments over the last ten years (1996-present); and (5) An assessment of and projection towards future work. Looking back, we hope we can share the excitement of this epoch period in mathematics education and the contributions to it which came from across the globe. Since its inception at the 1976 International Congress on Mathematical Education (ICME) in Karlsruhe, PME has addressed three major goals all addressing the need to integrate mathematics education and psychology. While PME clearly has welcomed and thrived on multiple theories of psychology, beginning with Skemp’s (1978) The Psychology of Learning Mathematics, it has preferred those with a cognitive, and to some extent, an affective orientation. Two major theories of intellectual development have been dominant, namely constructivism and socio-cultural perspectives. In recent years, these two theories have intermingled, but in this volume, they are separated as we trace their paths, overlapping and distinctive. We will not give in to the frequent temptation to cast constructivism and socio-cultural perspectives as a diametrically opposed where one is personal/individual and the other social; but rather track the evolution of the theory via the theorists and the perspectives that they assign to their work.