This report provides a description of the development process, field testing, and psychometric properties of the fall 2015 grades 3-5 Elementary Mathematics Student Assessment (EMSA), a student mathematics test designed to be administered in a whole-group setting to students in grades 3, 4, and 5. The test was administered to 2,614 participating grade 3, 4, and 5 students in 266 classrooms located in 10 public school districts in Florida during fall 2015. Focused on number (including whole number and fractions), operations, and algebraic thinking, the student assessment was designed to serve as a baseline measure of student achievement for use as a covariate in a randomized controlled trial evaluating the impact of a teacher professional development program on student learning. [This report was prepared by the Florida Center for Research in Science, Technology, Engineering, and Mathematics (FCR-STEM) within the Learning Systems Institute.]