Mathematics education reformers have long called for improved learning opportunities for all students across P-12 classrooms in the United States (U.S.). Although instruction that promotes mathematics as sensemaking and problem solving has been recommended (National Council of Teachers of Mathematics [NCTM], 2000; National Research Council [NRC], 1989), this shift dramatically differs from the way many classroom teachers once learned and taught math (Hiebert, 1999). Thus, local school systems are left to determine how to create conditions that support changes in teachers' instruction (Hopkins et al., 2013). To address this challenge, many schools hire mathematics specialists (MSs) as they embody key features of effective professional development (PD;Gibbons & Cobb, 2017). Multiple models of PD (lesson study, professional learning communities [PLCs],