Singapore's officially bilingual education policy, in which the majority of children are schooled through a non-native medium with their "Mother Tongue" (an ethnic heritage language that is not necessarily spoken in the home) as a single school subject only, has resulted in dramatic language shifts in the population and high academic achievement as measured by international comparison studies. Much current second language acquisition theory would predict failure for such a policy. This paper examines the assumptions concerning language planning and second language acquisition underlying the city-state's language-in-education policy, their relation to current theory in the field, and how the case of Singapore can support or challenge these different theories.