Students all learn at different rates and need a varying amount of support to reach goals based on individual needs. Response to Intervention (RtI) incorporates individualized research-based strategies based on individual students needs to help promote academic growth and is set up in three tier levels. The purpose of this quantitative ex post facto study was to determine the following: the difference in growth rate of first- through fourth-grade students in Tier 2 RtI and the first- through fourth-grade students in Tier 1 RtI at three elementary schools, the student growth based on three different RtI implementation approaches, and the impact of RtI group size on student growth. Data were collected on 971 students using the scale score data from Star Reading benchmark assessments. The data were entered into a spreadsheet and analyzed in SPSS. For Research Questions 1 and 2, a two-way ANOVA was conducted. For Research Question 3, the data were analyzed using a factorial ANOVA. All data were analyzed using a 0.05 level of significance. This study determined the following findings to be statistically significant: RtI is most effective when implemented in the lower grades, and push-in instruction from an interventionist working with a small group of students in the classroom setting produced the highest gains in reading. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]