"Odyssey Math," published by CompassLearning[R], is a web-based K-8 mathematics curriculum and assessment tool designed to allow for instructional differentiation and data-driven decision making. The online program includes electronic curriculum and materials for individual or small group work, assessments aligned with state curriculum standards, and a data management system that allows teachers to develop individualized instructional and assessment tools, as well as track individual and classroom student performance. "Odyssey Math" can be used as a standalone curriculum or as a supplement to other mathematics curriculum. The primary school version of the Odyssey Math curriculum focuses on fundamental math skills like numeracy for the earlier grades, while in later grades, the curriculum equips students for skills necessary in middle and high school mathematics. The interactive activities used for both age groups allow for the application of ideas, tools, and manipulatives, and build upon previous knowledge. The What Works Clearinghouse (WWC) reviewed 14 studies of "Odyssey Math." One of these studies meets WWC evidence standards with reservations. The remaining 13 studies do not meet either WWC evidence standards or eligibility screens. Based on the one study, the WWC found potentially positive effects in math achievement. The conclusions presented in this report may change as new research emerges. Five appendices are included: (1) Study Characteristics; (2) Outcome measures; (3) Summary of study findings; (4) Rating; and (5) Extent of evidence. (Contains 10 footnotes. Appendices are footnoted individually.) [The following study is reviewed in this intervention report: DiLeo, J. (2007). "A study of a specific language arts and mathematics software program: Is there a correlation between usage levels and achievement?" Unpublished doctoral dissertation, Indiana University of Pennsylvania, Indiana, PA. For the "Odyssey Math. What Works Clearinghouse Intervention Report" on Middle School Math, dated March 2009, see ED504602.]