Two things are perhaps clear from this chapter. The first is that well-conducted international comparisons of student achievement in science and mathematics, and TIMSS in particular, are a rich source of data about science and mathematics education in many countries. There is much that is yet to learned from the outcomes of these studies, especially when they are examined in fine detail and considered alongside other relevant findings, e.g., those relating to school improvement and effectiveness. The second is that international differences in student achievement in science or mathematics are much less important than a detailed understanding of what lies behind them, i.e., how they can be explained. Establishing such explanations is difficult. It requires extreme sensitivity to the social, cultural and historical contingency of education systems and what goes on within them. It is equally necessary to avoid confusing correlation with causality but that there is much to be gleaned seems incontestable.