This article describes and discusses two case studies of content-based instruction for second language education — foreign/second language immersion for majority language students in Canada and dual language education for minority language students in the U.S. After discussing the rationale for CBI in general, it examines 45 years of research on each program model and provides empirical evidence on a number of important issues, including: students’ proficiency in the two languages used for instruction; non-language academic outcomes; whether age is an important factor in students’ language outcomes; and the relationship between age of first exposure to the second language and outcomes in that language. Two outstanding major issues are discussed at some length; namely, the suitability of these programs for at-risk learners and the need for a coherent model of how best to integrate language and content instruction to maximize second language proficiency without detracting from academic achievement. Suggestions for future directions are provided.