"Odyssey® Math" is a web-based program developed by Compass Learning® for mathematics instruction in grades K-8. The online program includes a mathematics curriculum and formative assessments designed to support differentiated and data-driven instruction. Based on assessment results, the program generates an individualized sequence of mathematics topics and skills-a "learning path." "Odyssey® Math" is often used as a prescriptive tool, where students can start by taking a diagnostic assessment aligned with local or state standards. Teachers can modify learning paths to match their lesson plans or to align them with district scopes and sequences. The What Works Clearinghouse (WWC) identified three studies of "Odyssey® Math" that both fall within the scope of the Primary Mathematics topic area and meet WWC group design standards. Two studies meet WWC group design standards without reservations, and one study meets WWC group design standards with reservations. Together, these studies included 2,768 students in grades 4-8 in 41 schools. The WWC considers the extent of evidence for "Odyssey® Math" on the mathematics achievement of primary students to be medium to large for the mathematics achievement domain, the only domain examined for studies reviewed under the Primary Mathematics topic area. (See the Effectiveness Summary on p. 4 for more details of effectiveness for the mathematics achievement domain.) The following are appended: (1) Research details for DiLeo (2007); (2) Outcome measures for the Mathematics Achievement Domain; and (3) Findings Included in the rating for the Mathematics Achievement Domain. A glossary of terms is provided.