In June of 1998, Proposition 227 was passed by 61 percent of the California electorate. The initiative was intended to significantly alter the ways in which the state's English learners (ELs) are taught. Proposition 227 requires that ELs be taught "overwhelmingly in English" through sheltered/structured English immersion (SEI) programs during "a temporary transition period not normally intended to exceed one year," and then transferred to mainstream English-language classrooms. In 2000, the California Department of Education contracted with the American Institutes for Research (AIR), assisted by WestEd, for a five-year, legislatively mandated evaluation of the effects of the Implementation of Proposition 227 on the Education of English Learners. A combination of student achievement analyses, phone interviews, case study site visits, and written surveys was used to examine such questions as how the proposition was implemented, which EL services are most and least effective, and what unintended consequences resulted from Proposition 227's implementation. This is the culminating report, synthesizing findings from research performed across all five years of the study and submitted to the California Department of Education. There is a particular focus in this report on the research conducted in Year 5. Appended are: (1) Project Staff and Organization; (2) Chapter III Methodological Notes; (3) Chapter IV Exhibits; (4) Chapter V Exhibits; and (5) Chapter VI Exhibits. A glossary is also included.