There are two camps of thought on the early education of English Language Learners (ELLs) in the United States: i) They should be taught primarily in their native language with a gradual increase in the amount of English in their curriculum over the course of Kindergarten through 5th grade, or ii) They should be fully immersed in English immediately, with only ancillary support in their native language for particularly difficult academic concepts. When California voters passed Proposition 227 in 1998 they effectively agreed with the latter theory and banned bilingual education in public elementary schools for children who are not yet proficient in English.

This thesis investigates the apparent effectiveness of Two-Way Spanish Immersion in English language acquisition by analyzing aggregate results of samples taken from two distinct populations of Public Elementary Schools in California: schools that are exclusively Two-Way Spanish Immersion (\textgreater50\% of their curriculum is taught in Spanish), and schools that are exclusively Structured English Immersion and/or English Language Mainstream. This analysis uses a sampling design that controls for confounding factors such as income and parent education level.