M y 1980 article argued that cognitive/academic language proficiency (CALP) can be empirically distinguished from basic interpersonal communicative skills (BICS) such as accent, oral fluency, and sociolinguistic competence in both first and second languages (L1 and L2), and that cognitive/academic proficiencies in both L1 and L2 are interdependent, that is, manifestations of a common underlying proficiency. These propositions were then applied to the interpretation of data relating to (a) age and L2 learning and (b) bilingual education. Specifically, older school-age students make faster progress in absolute terms than younger students in acquiring L2 academic proficiency because they can apply their better developed CALP to L2 learning. With respect to bilingual education, I used the common underlying proficiency construct to explain why less instructional time through the majority language results in no adverse academic consequences in that language. In fact, for students whose L1 is a minority language, the trend is toward an inverse relationship between achievement and instructional time through the majority language (e.g., Francis, Lesaux, & August, 2006). The common underlying proficiency makes possible transfer of concepts, skills, and learning strategies across languages.