This study addresses the effectiveness of "Everyday Mathematics" (EM), a widely used core mathematics curriculum that reflects over two decades of National Science Foundation (NSF)-sponsored research and development studies (Klein, 2007; National Research Council, 2004) and aligns well with recommended policies and practices by the National Council of Teachers of Mathematics (NCTM) Curriculum Focal Points (2006) and National Mathematics Advisory Panel (NMAP) (2008). This and other similar curricula are increasingly needed to strengthen student math skills and ensure that all children are at or above grade level in math proficiency. Statistics present concerns to reaching this goal--only 42% of fourth grade students and 35% of eighth grade students are at or above proficient-level on National Assessment of Educational Progress (NAEP) math scores (National Center for Educational Statistics, 2013). Recent initiatives emphasize the role of early math instruction and curricula, recognizing that students need to develop a well-defined set of skills in critical math content areas (e.g., whole numbers, fractions, elements of geometry, and measurement) in early grades in order to prevent difficulties in later grades. The NMAP (2008) recommended that all students receive effective preparation from an early age to ensure their later success in algebra and emphasized the need for early math programs that mitigate and prevent difficulties. To date, however, Everyday Mathematics is only one of a few elementary school math curriculum reviewed by the What Works Clearinghouse (WWC) (2007) that has demonstrated "potentially positive effects," with the evidence for effectiveness on math achievement rated as medium to large. The "Everyday Mathematics" (EM) curriculum, developed by the University of Chicago School Mathematics Project (UCSMP) during the mid-1980s, and published by Wright Group/McGraw-Hill, relies on research-based practices, which according to the National Research Council (2004), are supported by more researchers and empirical studies than any other elementary mathematics curriculum. EM is a PK-6 curriculum emphasizing six content strands (numbers and numeration; operations and computation; data and chance; measurement and reference frames; geometry; and patterns, functions, and algebra) with learning targets or curriculum focal points identified for each of the six strands at each grade level. Developed largely from NSF-sponsored studies, the curriculum shows promise in preventing math difficulties in early grades. It emphasizes a constructivist philosophy, with a strong emphasis on real-life problem solving, manipulatives, concept development, and targeted use of technology and parent participation. Besides being the highest rated elementary school math curriculum in terms of effectiveness by the WWC (2007), it has also been widely adopted and used across the country. EM is used by nearly 4 million students in more than 11,000 schools in more than 3,000 districts in all 50 states (SRA/McGraw-Hill, 2009). Results from quasi-experimental evaluations have revealed statistically significant math achievement advantages of +0.16 relative to other math curricula (Carroll, 1998; Carroll & Isaacs, 2003; Riordan & Noyce, 2001; SRA/McGraw-Hill, 2003; Waite, 2000; WWC, 2009; Woodward & Baxter, 1997). In addition, a cluster randomized controlled trial (RCT) currently being conducted by the publisher is documenting potential impacts of EM on math achievement in grades K-5 students in three schools in Washington D.C. The study is assessing classroom-level impacts of treatment assignment on the TerraNova math achievement test scores (SRA/McGraw-Hill, 2009b). Despite the curriculum's widespread use and promising research findings, EM has not been evaluated rigorously on a large scale as part of an objective, third-party evaluation. This study provides preliminary evidence that the impacts of EM are not significant on overall students' mathematics performance when implemented at scale in a large sample of schools after one or two years relative to other core math curricula. Tables are appended. [SREE documents are structured abstracts of SREE conference symposium, panel, and paper or poster submissions.]