Rising numbers of English learners (ELs) being placed in general education high school mathematics classrooms with underprepared teachers is an increasing concern in U.S. public schools. Despite its importance, there has been little research on how to best support high school mathematics teachers in their attempt to teach both the highly specialized language and content of mathematics. To ensure equitable access to high-quality education for ELs, it is of vital importance to understand how best to educate and support teachers to successfully instruct ELs. The purpose of this mixed methods multiple case study was to investigate and address the perceived needs of high school mathematics teachers in how to better support ELs in general education classrooms. Through the use of a focus group, the perceived needs of high school mathematics teachers of ELs were established, learning modules were created to address the needs, and participants were then observed pre- and post-completion of the online learning to determine if the modules had any impact on teaching practices. The study demonstrated that the learning modules had a positive impact on the instructional strategies of all four participants, two at a statistically significant level. Participants embraced vocabulary instruction, and created other small shifts in their classroom to allow ELs to more fully engage in learning. Additionally, participants found the modules engaging and directly applicable to their classroom. In the future, additional professional development should put more of an emphasis on individualizing accommodations and language objectives for each student, rather than focusing on strategies that are interpreted as being more universally beneficial. Emphasis should also be placed on how to incorporate support for ELs who are at the upper end of English language proficiency levels. While these students are approaching English fluency, their unique needs must still be fully supported and acknowledged. [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.]