"Saxon Math," published by Houghton Mifflin Harcourt, is a core curriculum for students in grades K-5. A distinguishing feature of the curriculum is its use of an incremental approach for instruction and assessment. This approach limits the amount of new math content delivered to students each day and allows time for daily practice. New concepts are introduced gradually and integrated with previously introduced content so that concepts are developed, reviewed, and practiced over time rather than being taught during discrete periods of time, such as in chapters or units. The What Works Clearinghouse (WWC) identified 26 studies that investigated the effects of "Saxon Math" on the math performance of elementary school students. The WWC reviewed 14 of those studies against group design evidence standards. One study (Agodini, Harris, Thomas, Murphy, & Gallagher, 2010) is a randomized controlled trial that meets WWC evidence standards without reservations, and one study (Resendez & Manley, 2005) is a quasi-experimental design that meets WWC evidence standards with reservations. Those two studies are summarized in this report. Twelve studies do not meet WWC evidence standards. The remaining 12 studies do not meet WWC eligibility screens for review in this topic area. Appended are: (1) Research details for Agodini et al. (2010); (2) Research details for Resendez and Manley (2005); (3) Outcome measures for the mathematics achievement domain; (4) Findings included in the rating for the mathematics achievement domain; and (5) Summary of supplemental findings for the mathematics achievement domain. A glossary of terms is included. (Contains 5 tables and 10 endnotes.)