Elementary mathematics coaches are placed in schools to construct leadership roles and to provide on-site, collaborative professional development addressing mathematical content, pedagogy, and curriculum in an effort to enhance instruction and improve student achievement. This 3-year randomized control study found that over time coaches positively affected student achievement in grades 3, 4, and 5. In these grades, this significant positive effect on student achievement was not evident at the conclusion of the first year of placement of a coach in a school but emerged as knowledgeable coaches gained experience and as a school's instructional and administrative staffs learned and worked together. The coaches in this study engaged in a high degree of professional coursework addressing mathematics content, pedagogy, and coaching prior to and during at least their first year of placement. Findings should not be generalized to coaches with less expertise. (Contains 1 figure, 6 tables, and 3 notes.)