The National Statement on Mathematics for Australian Schools supports an increased emphasis on developing number sense through mental computation in recognition of the role of the calculator. The Calculators in Primary Mathematics project is a long-term investigation into the effects of the introduction of calculators on the learning and teaching of primary mathematics. The project commenced in 1990 at prep and grade 1 level, with 45 prep to grade 3 classes in six schools participating in 1992. All children in the project are "given" their own calculator to use whenever they wish. Their teachers are provided with systematic professional support, although they are not provided with curriculum materials or classroom activities. This paper focusses on changes in teachers' expectations of children's mathematical performance and curriculum issues arising from these changes.