Background: 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). Purpose: This paper reviews the extant research on SEI, its definitions, origins, and its effectiveness, particularly in contrast to other instructional strategies. This paper asks, Does the research bear out the Court's conclusion? What is the evidence that Arizona's program of SEI is really superior to other approaches, including bilingual or dual language education? How are Arizona's EL students faring under this "significantly more effective" instructional program? Research Design: Data on the relative effectiveness of SEI are drawn from a comprehensive review of the literature. Analysis of public documents, particularly records from the Arizona English Language Learners Task Force, which was charged with selecting a research-based instructional program for English learners. Drawing from a recent ethnographic study and student achievement data, we examine the impact of structured English immersion programs on English learners in Arizona thus far, beginning with achievement outcomes. Conclusions/Recommendations: There is no research basis for the Court's statement the SEI is "significantly more effective;" 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 a sole focus on English instruction, 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.