Enormous resources have been devoted to international assessments of mathematics and science over the past 50 years. This chapter presents a summary of what the field has learned from all of these studies to clarify ways in which schools really do matter. The focus is on the educational policy-relevant insights gleaned from a macro-level view. In addition, it explores some of the cross-country results that have emerged particularly from the more current studies of PISA and TIMSS (both TIMSS-95 and TRENDS). Ten key findings are summarized that reflect what has been found that has international implications rather than those that are country-specific. In conclusion, results from three countries, Russia, Germany and the United States, are addressed as the effects related to opportunity to learn are broad based, have international implications, and the results have been published so that the results are accessible beyond national boundaries.