Other things being equal, students in smaller classes have higher achievement. Why does this happen? The mathematics lessons of 63 Year 5 teachers in government schools were observed over a term, and their detailed teaching practices were recorded. The teaching practices which varied for classes of different size, and formed part of the link between class size and mathematics achievement, involved class grouping, the number and type of interactions between the teacher and students, teacher ques- tioning, and homework practices. A causal model linking student, school and teacher background information, class size, teaching practices and mathematics achievement was developed and tested.