In 2010, 6% of students in the United States were performing at an advanced level in mathematics. During the previous year, the United States produced over 987,000 low math achievers. The theoretical foundations of this research project study were found in Cognitively Guided Instruction (CGI). The research problem addressed in this study is the unknown influence of CGI on the mathematics achievement of 2nd grade students, as measured by the Criterion-Referenced Competency Tests (CRCT). The purpose of this study was to measure the impact of CGI on CRCT achievement scores of 104 students who had been administered the test from 2007 to 2010. A quasiexperimental design with a nonequivalent control group design and pretest and posttest scores was used in this quantitative study. Of the sample, the treatment group included 53 students, while the control group consisted of 51 students. Using ANCOVA, the results of this study revealed a significant difference between math scores from the treatment group (M = 841.09) and the scores of students from the control group (M = 820.80). These findings illustrated the merits of CGI on gains in mathematical achievement. CGI was found to be instrumental in improving instruction, which may have a positive influence on student learning and the school environment, as well as improving mathematics understanding nationally. Specifically, positive social change may happen as teachers develop ways of engaging students, which lead to increased mathematical understanding. As the dynamics of classroom social communication changes, children learn to think and act mathematically. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]