Academic English (AE) refers to the language used in school to help students acquire and use knowledge. This article reviews current literature to determine what is known about the nature of AE within the context of K–12 schooling. It describes how AE is conceptualized in the education research literature, how these conceptualizations are realized in instructional practices, and the implications of these conceptualizations for teacher education and professional development. The student population that is the primary focus of the article is English language learners, but the findings have implications for all students who struggle with learning AE. The article raises critical challenges in defining and operationalizing AE for instruction and suggests areas for further inquiry.