The purpose of this article is to present a brief synopsis of major theoretical orientations of second language acquisition (SLA) processes and offer practical implications for middle school teachers who work with English language learners (ELLs). ELLs' academic needs in middle school settings are particularly unique. Compared to younger children, middle school ELLs face more challenging tasks. They need to develop both complex content knowledge and language skills at the same time to be successful in school. Given that ELLs' language and literacy development is foundational to their successful academic learning across the curriculum, it is important for middle school teachers to deepen their theoretical understanding of ELLs' language and literacy learning processes. Yet, many teachers experience difficulties in understanding SLA theories because the conceptually complex theories are often presented in an inaccessible manner, obscured by linguistic jargon. This article, drawn from a review of theoretical perspectives in the literature and over a decade of classroom studies, provides a synopsis of the major theoretical orientations. These theories provide important instructional implications on how middle school teachers can successfully support ELLs' language and literacy learning across the curriculum.