Among the many options practiced in second language education around the world, a model that has attracted increasing attention is one in which a second language is used as the medium of instruction in schools. The term “immersion education” was adopted in the 1960s to describe such programs in the context of English-speaking children being taught entirely through the medium of French in selected schools in Quebec, but can be used to describe a variety of situations in which a home-school language switch occurs for a variety of reasons or purposes. These include immersion for majority students in a minority language to enhance second language learning (e.g., for Anglophone students in Quebec), immersion for language support and language revival (e.g., to promote Catalan among Spanish-speaking children in Spain), and immersion in a language of power to facilitate English-medium education (e.g., for Cantonese speakers in Hong Kong). As Johnson and Swain demonstrate, such cases of immersion education differ from other forms of bilingual education as a consequence of factors such as the role of the L2 as a medium of instruction, the nature of the immersion curriculum, the level of support available for the L1, the attempt to achieve additive bilingualism, the fact that L2 exposure is largely confined to the classroom, the students' limited or nonexistent proficiency in the L2 on entry into the program, and the bilingual status of the teachers.