This article is a report on the findings of a case study that focuses on a first grade teacher's noticing of children's understanding of linear measurement along a learning trajectory, extending Jacobs and her colleagues' framework (Jacobs, Lamb, & Philipp, 2010). It documents what the teacher noticed in terms of attending to and interpreting student strategies in four different contexts during her participation in a lesson study. The findings indicate that the teacher was overall more successful in attending to student strategies than interpreting mathematical understanding reflected in the strategies when she used a learning trajectory as a tool to notice student understanding. More interestingly, we found that her level of noticing differed depending of the role that she took in the process of lesson study. [For the complete proceedings, see ED584829.]