Improving children's "number sense" has been a core component of recent mathematics curricular and instructional reforms (CCSS, 2010; NCTM, 2000, 2014) in the United States. In support of this goal, mathematics educators have developed a variety of instructional practices designed to improve students' "number sense." One increasingly popular instructional routine is "Number Talks." Developed originally by Kathy Richardson and Ruth Parker and popularized by Sherry Parrish and Jo Boaler, Number Talks are 10-15 minute daily exercises where students practice solving problems without pen or paper. By flexibly solving problems and learning from other students' strategies, Number Talks are claimed to increase students' "number sense" (Boaler, 2015; Boaler et al., 2018; Sun et al., 2018). However, researchers and educators have not measured this hypothesized growth in student "number sense" in any systematic way. Therefore, the goal of this project was to develop a measure of student number sense with valid and reliable scores that can be used to assess the impact on student number sense of instructional practices such as Number Talks. The measure development process and validation results from a sample of 992 5th-12th grade students are presented here.