"To whom do you turn in this school for advice or information about mathematics instruction?" (Spillane, Healey, and Parise 2009, p. 413). When teachers in forty-four schools were asked this question, they were more likely to indicate a teacher leader in their school, rather than the school's principal or any other administrator. Who are these teacher leaders for mathematics, and what do they do? In many schools, such leaders are knowledgeable, on-site teachers who support their colleagues' efforts to interact about all facets of mathematics teaching--curriculum, lesson planning, student work, assessments, and school improvement. Because these teachers are facilitating on-site, job embedded, professional learning, many school districts are formalizing this practice and creating either elementary mathematics specialist or coaching positions with the hopes of developing these leaders. The two positions are somewhat different. Elementary school mathematics coaches focus on working with individual teachers to foster instructional change; specialists are also expected to advance a school's mathematics program. Are mathematics specialists or coaches merely the latest "grand promise" in education, or do they really offer a path toward improved instruction in schools? For many readers, the answer to that question depends on whether these teacher leaders influence student achievement. A recent three-year study of elementary mathematics specialists examined that issue (Campbell and Malkus 2011). A primary focus of this study was to determine the impact of mathematics specialists on student achievement (in grades 3, 4, and 5) as measured by the standardized assessments that were administered by one state to meet No Child Left Behind federal regulations.