For the lay-person, ‘words’ are the linguistic units which they recognise intuitively and which seem to them to be the most closely synonymous with ‘language’. Learners of any foreign language realise quite early on that the part of it they need to control the most effectively is its vocabulary. If their mastery of the pronunciation and grammar of the target-language is quite sound, they will not get very far, communicatively speaking, without knowing a reasonable number of words. Linguists, on the other hand, have a rather perverse tendency to locate lexicology outside the central core of their discipline, probably because the thousands of words in a language cannot be embraced by ‘rules’ in the same way as its phonological or grammatical systems. However, in this book, which is designed to take the reader from the situation of the layman to that of the linguist, we will begin with vocabulary before looking at the other levels of language structure.