Recent work on differences in mathematics achievement has highlighted the importance of classroom, teacher and school factors. The present study used data from the Third International Mathematics and Science Study (TIMSS) to look at student, classroom and school factors influencing mathematics achievement in Australia and the United States (US). It found that classroom differences account for about one-third of the variation in student achievement in the US and over one-quarter in Australia. Most of the classroom variation in both countries was due to compositional and organisational factors, very little of it due to differences between teachers. This has important implications for policy regarding the improvement of mathematics achievement. It suggests that school systems may gain little by targeting teachers only, and need to give consideration to the role of pupil grouping practices and the effects of tracking and streaming on classroom learning environments.