In the Horne v Flores Supreme Court decision of June 25, 2009, the Court wrote that one basis for finding Arizona in compliance with federal law regarding the education of its English learners was that the state had adopted a "significantly more effective" than bilingual education instructional model for EL students --Structured English Immersion (SEI). This paper reviews the extant research on SEI, its definitions, origins, and its effectiveness, particularly in contrast to other instructional strategies. The paper concludes that there is no research basis for the Court's statement, that at best SEI is no better or no worse than other instructional strategies, particularly bilingual instruction, when they are both well implemented. However, SEI as implemented in Arizona carries serious negative consequences for EL students stemming from the excessive amount of time dedicated to it, the de-emphasis on grade level academic curriculum, the discrete skills approach it employs, and the segregation of EL students from mainstream peers. Moreover, the paper argues that there are, in fact, strategies that can ameliorate these problems as well as provide an additive, rather than a subtractive, educational experience for English learner and mainstream students alike. (Contains 1 figure and 100 footnotes.)