The nine major empirical studies of Lexical Issues in Language Learning address key issues in the development and use of vocabulary by child bilinguals and older second language learners. The thematic focus in this collection of Language Learning articles is on the assessment of lexical development in bilinguals at different points in the lifespan; the psycholinguistic factors that determine the learnability of second language lexis; and the conditions on communication tasks that promote the learning and retention of second language vocabulary or lead to different strategies for handling lexical problems. The studies address learning in diverse situations internationally, utitlize innovative research methods conducted with the highest levels of expertise, and concern acquisition in various languages: Chinese, English, French, German, and Spanish.