The influx of people migrating to new lands for opportunities has been an issue of increasing complexity in recent years. Classrooms are now more diverse than ever. Studies through the National Association for Educational Progress found that English Language Learner (ELL) students do not achieve at consistently high levels when compared to other students on standardized mathematics exams. This book addresses how the instructional practices of high school mathematics teachers impact ELL students. The purpose of this evaluative case study approach was to explore the extent to which the implementation of SIOP influenced mathematics instruction in a mid-sized rural high school. The conceptual framework for this study was formed by combining Krashen‘s i+1 nativist theory for language acquisition through comprehensible input, Long‘s interactionist theory for acquisition of knowledge and Bandura‘s teacher efficacy theory. The results of this study could enhance the capacity of mathematics teachers to teach their second-language learner needs. Implications for positive social change include removing language barriers so that more students may continue taking advanced mathematics courses.