The author reviews both theory and research concerning learning foreign language lexis, and reports on a study into the dynamics of advanced learners’ L2 lexicons. The theoretical part offers a discussion of the concept of the bilingual mental lexicon, the notion of lexical competence, the interface of word knowledge and the language skills, and the current trends in vocabulary testing. The study described in the book, which adopts a unique longitudinal design, is a significant contribution to our understanding of the changes taking place in receptive and productive lexicons in time. The obtained results carry important implications for course design, instruction and testing procedures in the context of teaching English as an L2 at the advanced level.