Educators are concerned with the academic attainment of English learners (ELs) in U.S schools, as this student population's numbers continue to grow. In 2014-15, 4.6 million, or about 9%, of all public school students in the United States were ELs. The number of ELs in public schools is projected to represent 25% of all public school students by 2025 (National Center for Education Statistics, 2017; National Clearinghouse for English Language Acquisition, 2013). The purpose of this study was to examine non-ESOL elementary content area teachers' perceptions of the online SIOP® professional development course regarding implementing the six features of lesson preparation and applying the eight components of SIOP® within their lesson preparation. The study was guided by three research questions and an online survey to obtain teacher perceptions regarding their implementation of the key SIOP® components in lesson preparation, application of the eight SIOP® components, and potential ways that SIOP® had positive effects on instructing ELs. Qualtrics (a web-based tool) was used to create the descriptive survey and generate reports from the participants' responses. From the research findings, recommendations were made to contribute to the literature and for future study in general, for the school district, and the researcher. SIOP® as an online professional development tool has the potential to reach a growing audience of content teachers who require best practices and sound approaches to teaching ELs in their classrooms. [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.]