Though it is estimated that about half of the world's population is bilingual, the estimate for the United States is well below 20% (Grosjean, 2010). Amid growing recognition of the need for second language skills to facilitate international commerce and national security and to enhance learning opportunities for non-native speakers of English, many U.S. public, charter, and private schools have developed dual-language immersion (DLI) programs. The goal of these programs is to help the growing number of language minority students learn English and achieve academically, while giving language majority students the opportunity to develop proficiency in another language. Though numerous studies have established a positive relationship between dual-language education and student achievement, important questions about the relationship between the two remain. The research questions addressed in the paper are as follows: (1) What is the causal effect of dual-language immersion education on student achievement in mathematics and reading in grades 3 through 8, on student attendance in grades K through 8, and on initial English language learners' exit from ELL status in grades 1 through 8? (2) Does this effect differ for English language learners versus native speakers of English or by instructional model (90 versus 50 percent of time in the partner language)? (3) What does it cost to implement dual-language immersion programs relative to non-immersion programs? What are the components of these costs, and do they vary by instructional model? Findings to date suggest that immersion may improve student achievement in reading (in English) without diminishing other performance. The following are appended: (1) References; and (2) Tables and Figures.