The need for practicing teachers to reflect and build on their knowledge of mathematics for effective practice, and ongoing professional development is well recognised by the educational community. In this chapter I examine two dimensions of this knowledge (content and pedagogical content knowledge) as reported by two teachers who are new to the profession. The knowledge dimensions were examined by using a mobile technology tool (smartphone) to reflect on the design and teaching of fractions. Results indicate that the participants were competent users of smartphones and displayed a wide repertoire of the knowledge that was relevant for teaching of fractions in the primary curriculum. The smartphone appears to be a useful device for mediating teachers’ content and pedagogical content knowledge.