Virtually all nations of the world today realize the research and policy value of student performance data that come from testing the cognitive skills of students. While there is wide variation across nations in testing - differing by subject matter, grade level, purpose, and quality of testing - the idea of assessing what students know as opposed to how long they have been in school has diffused around the world, in part at the instigation of international development and aid agencies. Somewhat less known is that comparative cross-national testing has been going on for a long time. Nations participated in common international assessments of mathematics and science long before they instituted national testing programs. These common international assessments provide unique data for understanding both the importance of various factors determining achievement and the impact of skills on economic and social outcomes.