This Information Capsule summarizes the research on Structured English Immersion (SEI) programs. Most school districts use either the SEI or Bilingual Education (BE) model to instruct elementary level English Language Learners (ELLs). In SEI programs, ELLs receive all or most of their instruction in English. BE programs provide instruction in two languages -- the target language (English) and the student's native language. Studies comparing the effectiveness of SEI and BE programs have produced conflicting findings. There is, therefore, no consensus among researchers regarding the superiority of one instructional approach over the other. One notable exception is Canada's Early French Immersion (EFI) program that teaches elementary English-speaking students in French. Studies have found that elementary students in EFI programs acquire higher levels of French language proficiency than students enrolled in BE programs and attain mastery of academic skills at a level that is equivalent to that of students studying in all-English programs by the end of elementary school. Some researchers have concluded that the quality of the language acquisition program is more important than the instructional approach used to teach the language. In addition, they note that there is no "one-size-fits-all" model of instruction for ELLs. The characteristics of students, classroom teachers, schools, and local communities all affect program success. Researchers have identified characteristics of successful elementary level SEI programs. These characteristics, including high-quality teachers, clearly defined student entry and exit criteria, and grouping of students by level of English language proficiency, are summarized in this report.