What words come into your head when you think of SUN? For native English speakers, the most common responses are MOON, SHINE and HOT, and about half of all native speaker responses to SUN are covered by these three words. L2 English speakers are much less obliging, and produce patterns of association that are markedly different from those produced by native speakers. Why? What does this tell us about the way L2 speakers' vocabularies grow and develop? This volume provides a user-friendly introduction to a research technique which has the potential to answer some long-standing puzzles about L2 vocabulary. The method is easy to use, even for inexperienced researchers, but it produces immensely rich data, which can be analysed on many different levels. The book explores how word association data can be used to probe the development of vocabulary depth, productive vocabulary skills and lexical organisation in L2 speakers.