A History of Mathematics Education during the Twentieth Century describes the history of mathematics education in the United States with conceptual themes concerning philosophy, mathematics content, teacher education, pedagogy, and assessment. Each decade of the twentieth century is analyzed using historical documents, within the context of the aforementioned themes, to create a concise history of mathematical reform as it relates to history within the United States. Finally, conclusions are drawn as to which reform movements are similar and different throughout the century—depicting which aspects of reform can be seen again. Mathematics education tends to swing on a pendulum from "traditional education" including teacher-directed instruction with an emphasis on computation skills to "reform education," including student-directed instruction with an emphasis on problem solving. All decades are analyzed to see where they were on the pendulum and what aspects may have contributed to the current reform movements led by the Standards movement.