The recent releases of two major international comparative studies that addressed mathematics--the Trends in International Mathematics and Science Study 2003 (TIMSS 2003) and the Program for International Student Assessment 2003 (PISA 2003)--provide opportunities and challenges for mathematics education researchers interested in using the findings, instruments, and conceptual and theoretical perspectives of these studies as catalysts for secondary analysis and additional research. In particular, a number of important questions in mathematics education in the United States can be pursued, using various resources from these studies as a base, by mathematics education researchers and mathematicians collaborating with statisticians and assessment experts. We highlight some of the main findings of TIMSS 2003 and PISA 2003, and suggest how some of the instruments that were used in these studies, as well as the conceptual frameworks and priorities that guided them, might be beneficial in pursuing pressing questions in such areas as the role of curriculum in mathematics performance, the ways in which social contextual variables interact with mathematics learning, and the challenges in measuring the ability to use mathematics in real-world situations.