This study uses lesson study to investigate what mathematics teachers notice about students' mathematical reasoning during the planning of a lesson on fractions. Most research examines teaching noticing during or after a lesson, and focuses on the specificity of what teachers notice as a characteristic of noticing expertise. In this paper I propose a new notion of productive noticing, and apply it to analyse two vignettes of teachers' mathematical noticing during lesson preparation. Findings suggest that teachers' noticing is most productive when it goes beyond the specificity of what teachers notice to include justification based on what they have noticed about students' thinking. The study also demonstrates the usefulness of this construct in analysing what mathematics teachers notice when planning lessons. [For the complete proceedings, see ED597799.]