This paper presents the findings of an investigation into the influence primary teachers' knowledge of a researched-based framework describing children's cognitive development in early number has on their teaching practices. Survey and interview data from twenty-eight teachers were collected to determine teachers' perceptions of their understanding of the framework, their ability to use the framework to assess students' mathematical development and to plan appropriate instruction. The findings raise further questions about the influence of affective factors, such as teachers' confidence in their own knowledge, on their instructional decision-making. (Contains 1 figure and 1 table.) [For the complete proceedings, "Shaping the Future of Mathematics Education. Proceedings of the Annual Conference of the Mathematics Education Research Group of Australasia (33rd, Freemantle, Western Australia, Australia, July 3-7, 2010)," see ED520764.]