This article addresses the question of what language proficiency (LP) is, both theoretically and empirically. It does so by making a distinction, on one hand, between "basic" and "higher language cognition" and, on the other hand, between "core" and "peripheral components" of LP. The article furthermore critically examines the notion of "level" in most second language (L2) assessment scales, showing that it is confounded with people's intellectual functioning because higher levels of LP cannot be attained by people with lower intellectual, educational, occupational, or leisure-time profiles. It is probably for this reason that the Common European Framework of Reference for Languages (Council of Europe, 2001) fails to consistently distinguish between L2 development and L2 proficiency. The LP construct presented in this article can account for the fact that L2 learners with higher intellectual, educational, occupational, or leisure-time profiles may perform, at a given point in time, both better (i.e., in the domain of higher language cognition) and more poorly (i.e., in the domain of basic language cognition) than native speakers with lower profiles. While offering a research agenda for investigating individual differences in first language and L2 acquisition, the article also presents several implications for L2 assessment. (Contains 8 footnotes and 4 figures.)