learners can benefit from "sheltered instruction" in English, lessons in science or social studies that are tailored to their level of second-language proficiency. Gradually, they acquire the complex English skills required to make a successful transition to the mainstream - typically by the fourth or fifth grade.The key point is that language acquisition is a natural, developmental process that cannot be rushed. Indeed, placing children in incomprehensible classrooms and drilling them in meaningless exercises is likely to slow them down.Still, hopes for a shortcut to English die hard - as shown by Californians' vote to mandate a one-year "English immersion" approach that has no support in educational research. Critics say bilingual education has no support either, citing mixed results in program evaluation studies. It is true that such findings are less clear-cut than conclusions from basic research on second-language acquisition. Scientific comparisons of program effectiveness, which must track student outcomes over several years and control for differences among experimental groups, are difficult to design and costly to execute. Relatively few such studies exist. The best examples, however, confirm hypotheses about the benefits of bilingual instruction.Last year, using a sophisticated technique known as meta-analysis, a University of Texas study reviewed the scientific literature and found a small but significant edge for programs that used students' native language over those that did not. Researcher Jay Greene determined that the more rigorous the evaluation, the more support it provided for bilingual education. 4 This was true even though many of the comparisons were short-term and little was known about program design.The last large-scale evaluation, released by the U.S. Department of Education in 1991, took a more controlled approach. It compared the outcomes of three distinct program types: English-only immersion, quick-exit (transitional) bilingual education, and late-exit (developmental) bilingual education. Students' academic progress, charted in nine school districts over a four-year period, was most dramatic in the program that stressed fluent bilingualism. In the late-exit model, their achievement accelerated over time - almost catching up with that of English-speaking peers. In the other models, it leveled off well below national norms. 5 Wayne Thomas and Virginia Collier, of George Mason University, recently reported similar patterns in a larger group of school districts. They found that LEP children fared best in "two-way" bilingual programs, learning alongside English-speakers acquiring Spanish. 6 Thus far, however, the researchers have released limited data to support their conclusions - an issue that critics have been quick to seize upon.In 1997, the National Research Council condemned the "politicized" debate over language of instruction. It recommended greater emphasis on "finding a set of program components that works for the children in the community of interest, given the community's goals, demographics, and resources." 7 Nevertheless, the "what works" controversy is unlikely to subside anytime soon - at least not before policymakers support more extensive and rigorous studies comparing the range of program alternatives for LEP students.Footnotes1. California Language Census, summarized in James Crawford, Bilingual Education: History, Politics, Theory, and Practice, 4th ed. (Los Angeles: Bilingual Educational Services, 1998). 2. Stephen Krashen, "The Dropout Argument" (1998), available