Although learning English as a second language is ubiquitous across schools worldwide, it can be particularly challenging in classrooms with a linguistically diverse population of students. For example, although 76% of English learners in the United States speak Spanish as their native language, ELs in the United States actually speak more than 450 languages (Baker, Richards-Tutor, Gersten, Baker, & Smith, 2017). Moreover, all ELs, even Spanish-speaking ELs, are a remarkably heterogeneous group in terms of (a) English language proficiency, (b) native language proficiency, (c) socioeconomic status, (d) parental level of education, (e) country of origin, and (f) individual and family experiences (Durgunoglu & Goldenberg, 2011). Thus, understanding more nuanced ways to support this growing population of students should be a priority. Collectively, this book provides the most up-to-date review of our current knowledge about how the complexities of each of the linguistic registers across mathematics, science and social studies extends far beyond content-area vocabulary and warranting an intentional, purposeful focus on language, particularly academic English during content-area instruction. Moreover, the current disciplinary content standards demand the integration of discipline-specific language instruction within content-area instruction. To address these demands, the topics of the chapters in this book span content areas (ELA, mathematics, science, and social studies), topical areas (assessment, language growth, instruction, and professional development), and grade levels (preschool, elementary, and secondary). Each chapter provides a synthesis of the research on one of the specific topics, and it concludes with implications for practice and research. References to the most relevant research are provided