This book offers a comprehensive investigation into advanced students' knowledge of vocabulary in their L1 and L2. As a cross-sectional study, it examines the quantitative aspects of students' vocabulary knowledge through parallel tests of upper secondary level vocabulary, specialised vocabulary, and advanced vocabulary in both their L1 and L2. It also, primarily in qualitative terms, investigates students' L1 and L2 knowledge of polysemous words, lexical fields of near synonyms and false friends. Knowledge of derivative forms, idioms, proverbs, idiomatically used prepositions and multi-word verbs offer insights into both the breadth and depth of students' L1 and L2 vocabulary knowledge. Finally, it considers the extent to which students' results can be attributed to differences between inferencing skills in their L1 and L2. In each subfield, the pedagogical implications of the findings are discussed. This book will be of interest to teachers and researchers focusing on the teaching and learning of vocabulary.