This book presents a theory of what is called Basic and Higher Language Cognition (BLC and HLC), a theory aimed at making some fundamental issues concerning first and second language learning and bilingualism (more) empirical. The first part of the book provides background for and explication of the theory as well as an agenda for future research, while the second part reports on selected studies of language proficiency in native speakers, as well as non-native speakers, and studies of the relationship between literacy in a first and second language. Conceptual and methodological problems in measuring language proficiency in research on second language acquisition and bilingualism are also discussed. Further, the notion of levels of language proficiency, as rendered by the Common European Framework of Reference for Languages (CEFR), is critically examined, suggesting ways of empirically investigating a number of questions that the CEFR raises but is not capable of answering.