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 the United States (US) and Australia. It found that classroom differences account for about one-third of the variation in student achievement in the United States and over 0ne-quarter in Australia. Much of the classroom variation was due to compositional and organisational factors. This has important implications for policy regarding the improvement of mathematics achievement in schools.