The study investigated kindergarten to year three (K-3) teachers' conceptions of a constructivist approach to teaching and learning mathematics. It focused on three aims. The first aim was to investigate kindergarten to year three teachers' understandings of a constructivist approach to teaching and learning mathematics; the second, to identify methods K-3 teachers use to apply the approach in their mathematics classes and the third to determine whether the teachers' understandings of a constructivist approach influence their practice. Literature showed that teachers' conceptions or beliefs about a theory tend to influence their practice. The researcher undertook a field study to investigate the research problem. The study found that most K-3 teachers in the Newcastle Area of New South Wales have both a theoretical and practical knowledge of a constructivist approach which in turn influences their teaching.