The significant expansion of varying forms of student testing, while controversial in many countries, has not been generally linked to educational performance. Different testing regimes provide varying information to parents, teachers, and decision makers. We investigate how different types of information relate to student achievement. Our empirical analysis exploits data from over two million students in 59 countries observed across six waves of the international PISA test 2000-2015. Relying on the country panel feature of the data, we investigate how testing reforms relate to country performance on PISA tests over time, taking out country and year fixed effects. Expansion of standardized testing with external comparison, both school-based and student-based, is associated with improvements in student achievement. This effect is stronger in low-performing countries. By contrast, reforms to solely internal testing without external comparison and internal teacher monitoring including inspectorates are not related to changes in student achievement.