Abstract: Demand is constantly increasing for schools to improve the level of student performance as documented by standardized achievement tests. Currently the most sweeping reform of the public education system is the No Child Left Behind Act. This federal mandate requires schools to document adequate yearly progress as measured by state administered achievement tests. An important issue being considered by administrators and teachers is what impact developmentally appropriate practice (DAP) might have on those test scores. Even though some believe that DAP is best for young children, they are hesitant to implement these practices in the classroom. This hesitation is built upon the premise that their students may not do as well on standardized tests. The purpose of this study was to investigate the effects of DAP on student achievement in third grade classrooms in an urban school district in Provo, Utah, as evaluated during one academic year. Specifically, this study seeks to determine whether or not DAP practices impact the academic success of third-grade students, as measured by standardized tests. The hypotheses which guided this study proposed that students in more developmentally appropriate classrooms (MDAP) will achieve higher scores on achievement tests than students in less developmentally appropriate classrooms (LDAP). The DAP levels and measurements of each classroom were evaluated using the Assessment Profile for Early Childhood Programs. Achievement tests used in this study were standards based tests in Reading, Writing and Math. The findings in this research study revealed that students in MDAP classrooms scored higher than students in LDAP classrooms on all achievement tests. The relationship between DAP and reading and writing was somewhat more significant than the relationship with math. Implications for the profession and recommendations for further research are discussed.