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. Language Proficiency PERIPHERY CORE Linguistic cognition (knowledge and speed) in the phonetic-phonological, morphonological, morphosyntactic, and lexical domains: * Basic language cognition * Higher language cognition Metacognitive competences * Metalinguistic knowledge * Knowledge of various types of oral and written discourse with their own characteristics * Strategic competences * โฆ