The existence of a global language proficiency factor is discussed. This factor, cognitive/academic language proficiency (CALP), is directly related to IQ and to other aspects of academic achievement. It accounts for the bulk of reliable variance in a wide variety of language learning measures. Three propositions concerning CALP are reviewed. (1) CALP can be empirically distinguished from interpersonal communicative skills such as accent and fluency in first language (L1) and second language (L2). (2) CALP proficiencies in both L1 and L2 are manifestations of the same underlying dimension. (3) Because the same dimension underlies CALP in both L1 and L2, older learners, whose proficiency is better developed, will acquire L2 CALP more rapidly than younger learners. The relevance of this analysis for the concepts of semilingualism, code-switching, and bilingual education is outlined. Semilingualism is a manifestation of low CALP in both languages. CALP will be less active and effective when the L1 and the L2 are very dissimilar. In the presence of negative affective variables such as low motivation, CALP will not be applied to learning L2. If motivational involvement and adequate exposure to an L1 or L2 exist, CALP will be promoted in both languages regardless of which is the language of instruction. (PMJ)