This chapter examines large-scale comparative studies of mathematics education focussed on student achievement in an attempt to explain how such investigations influence the formation and implementation of policies affecting mathematics education. In doing so, we review the nature of comparative studies and policy research. Bennett’s (1991) formulation of policy development and implementation is used in examining national reactions to the results of international studies. Focus is given to the degree to which mathematics educators and others have played major roles in determining related policy outcomes affecting curriculum and the development and interpretations of the assessment instruments and processes themselves.