Eliciting and using student thinking (NCTM 2014) is a foundational pedagogical skill that informs the decisions teachers make during instruction. It informs the questions teachers pose, the focus of small-group and whole-class discussions, and even the small adjustments teachers make during a well-planned lesson. To use student thinking to strengthen the teaching of mathematics, a teacher needs to learn how to professionally notice students' mathematical thinking. This article discusses what it means to professionally notice students' thinking in a technology-mediated learning environment.