Foreign languages are currently a fundamental part of a person’s education. Governments invest many resources in teaching foreign languages, increasingly from younger ages. This chapter first documents the importance and economic benefits of foreign language knowledge. Second, it reviews some school programs in Spain and other countries that implement the learning of a second language through its use as the language of instruction. Third, it presents empirical evidence on the evaluation of these programs, explaining the methodological problems this entails and the solutions proposed in the literature.