This study examines how elementary preservice teachers notice children's mathematical thinking and how this noticing influences the evaluation of technological resources. In particular, we explore the aspects of thinking to which preservice teachers attend and how they interpret evidence about children's thinking when using the Spatial-Temporal Math (ST Math) program. Data collection included a group survey administered after an initial exploration of a set of ST Math activities, screencast recordings during which children used and talked about the program, and a reflective writing assignment. The findings of this study show how preservice teachers used their noticing skills (attending and interpreting) in their evaluations of the tool, in some cases prompting them to shift their evaluation on the basis of student thinking. [For the complete proceedings, see ED606556.]