The author explored the extent to which policy could be a source of sustainable professional development for elementary teachers' mathematical knowledge for teaching. The study involved three policies in England: the National Numeracy Strategy and the Primary National Strategy, introduced in response to concerns regarding mathematics teaching and student attainment, and a new National Curriculum. Professional development was offered to support teachers in the processes of changing their practice. This article focuses on the responses of eight teachers who suggest that the National Numeracy Strategy and the Primary National Strategy were useful sources of mathematical knowledge for teaching. However, the 2013 National Curriculum, along with the teaching for mastery policy, show how the teachers' professional development had been rooted in policy enactments and was temporal. The author discusses the need for sustainable professional development to facilitate teachers' continuous enhancement of their mathematical knowledge for teaching within a changing policy landscape.