The EdResearch for Action "Overview Series" summarizes the research on key topics to provide K-12 education decision makers and advocates with an evidence base to ground discussions about how to best serve students. This research brief breaks down what is known about multilingual students classified as English Learners (ML-ELs), how ML-ELs perform in K-12 education, and what challenges they face. Key insights provided include: (1) research-based practices--such as bilingual program models--district and school leaders can use to support the academic success and linguistic development of ML-ELs; and (2) one-size-fits-all practices to avoid that can limit many students' opportunities to engage with rigorous content. [This brief was produced in collaboration with the University of Texas at Austin, College of Education.]