Literally hundreds of scientific studies over the past half century have shown that bilingual education – when well designed and well implemented – is an effective approach for teaching second language learners. These findings have been consistent across numerous national borders and languages of instruction, all pointing to the conclusion that bilingual education "works."

To cite just one example, a major longitudinal study released in 1991 by the U.S. Department of Education found that the more schools developed children's native-language skills, the higher they scored academically over the long term in English. Students also benefited from acquiring fluency and literacy in two languages.

Of course, simply using two languages in the classroom is no panacea. Bad teaching is bad teaching in any language. What matters is how the languages are used – simply to provide translations or to stimulate students' cognitive and academic growth. What are the program's goals – a "quick exit" to the mainstream or the development of fluent bilingualism and biliteracy? Are teachers well trained to meet the needs of English language learners (ELLs)? Does the school provide adequate resources, materials, and support services? Are parents involved in their children's education? Is the program supported by the local school board and district administrators?

It is important to know that not all programs labeled "bilingual" are the same. Some feature only limited use of children's native language, often provided by teacher aides with limited training. Others set arbitrary limits on the length of time that children can spend in bilingual classrooms, transferring them to regular, all-English classrooms before they are ready. An increasing number of schools, under pressure from the No Child Left Behind Act, have narrowed the curriculum to focus exclusively on test preparation in language arts and math.

By and large, such programs have fared poorly in research studies as compared with programs that stress children's cognitive growth and bilingual abilities. Nevertheless, all these approaches are often lumped together in research designed to compare the effectiveness of bilingual education with all-English instruction for ELLs. In such cases, when good programs and bad programs are averaged together, the results are typically mediocre.

By contrast, when research is designed to test the theories underlying bilingual education, the programs have shown to be quite effective. Here are three recent examples:

Ramirez et al. (1991). This federally funded study traced the progress of more than 2,000 Spanish-speaking ELLs in nine school districts in five states over a four-year period. It found that students in developmental bilingual programs – which featured a gradual transition to English – significantly outperformed their counterparts in quick-exit, transitional bilingual programs and in all-English immersion programs when all three groups were tested in English.

Oller and Eilers (2000). The researchers compared 952 students in Dade County, Florida, enrolled in bilingual and English immersion programs. It reported that bilingual children scored higher in English literacy by 2nd grade – a gap that widened significantly by 5th grade.

Thomas and Collier (2002). Another federally funded study, this research confirmed the patterns reported by Ramirez et al. ELLs in Houston did better academically in programs that stressed native-language development. They fared best in two-way – a.k.a. dual immersion – programs in which English-speaking children learned Spanish alongside ELLs learning English.