From a consideration of variability in language-learner and language-user data, the concept of proficiency is considered in relation to models of second and foreign language learning. Proficiency is defined in relation to four separate dimensions: grammatical well-formedness, speech-act rules, functional elaboration, and code diversity. This provides a framework for considering two distinct models of language development: language learning viewed as movement along a developmental continuum, and language learning viewed as a non-developmental lectal continuum. These two models underlie the distinction between foreign language learning and second language learning. These are then related to contexts for language use that typically lead to four types or models of second and foreign language behavior: the instrumental, intergroup, integrative and interactional language models. These four functions are considered, and examples of their effects on the form of second and foreign language behavior are illustrated. Implications for second and foreign language learning theory and teaching methodology are discussed. (Author/AMH)