This article aims (a) to explore the operationalisation and definition of lexical L2 proficiency and related constructs with a view to identifying a set of measures that can adequately capture the dynamics of lexical L2 proficiency development over time, and (b) to shed more light on the development of lexical proficiency in French Foreign Language classes. After a discussion of theoretical, terminological and methodological issues in L2 vocabulary research, we present a longitudinal quantitative study of the lexical development of Dutch-speaking adolescents learning FFL in Dutch-medium schools in Brussels over a three-year period and compare these learners' lexical proficiency in French to native speaker benchmarks.