This study examines preschool teachers’ knowledge of their young students’ number conceptions and the teachers’ related self-efficacy beliefs. The authors found that promoting preschool teachers’ knowledge of appropriate mathematical tasks is interrelated with promoting their knowledge of students. The findings reveal that that teachers’ estimates of their students’ abilities increased as a result of participating in the program. The authors also saw that teachers’ improved the accuracy of their estimations related to students’ abilities to perform number-related tasks.