As numbers of English Learner (EL) students continue to grow from year to year, regular-education teachers are expected to have background knowledge necessary for supporting EL students at all tiers of the Response to Intervention (RtI) framework. To meet this challenge and provide high-quality instructional practices for ELs, the Sheltered Instruction Observation Protocol (SIOP) model was implemented in the district under study. The purpose of this study was to examine teachers' perceptions of the degree of implementation of the SIOP model within an RtI Framework. The population for this quantitative study comprised 92 teachers of regular education, English-language, and special education, who participated in the SIOP model training in one Midwestern district. A survey was used to gather data on participants' backgrounds, perceptions of degree of SIOP implementation within Tier 1, Tier 2, and Tier 3, and participants' responses to an open-ended question. The response rate was 35% (32/92). This study found that participants were implementing the features and components of the SIOP model within the three tiers of an RtI framework. However, certain features and components were more likely to be implemented by participants than others, regardless of area of certification, grade-level bands, and years since SIOP training. Effective professional development opportunities were found to be an essential component to support the implementation of SIOP within all three tiers of an RtI framework. A recommendation is that regular coaching opportunities include peer coaching study teams. Additionally, principals, coaches, and peer coach study teams must monitor the implementation of the SIOP framework within the RtI framework, as well as the potential impact on student outcomes. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]