The lexicon of Middle English has been approached from a variety of perspectives. The semasiological approach begins with the set of lexemes and investigates what they mean and what they meant at different times during the period in question. The lexicon has also been approached from the opposite direction: the onomasiological approach takes a set of objects or concepts as the starting point and investigates what words were used to express them. This chapter considers the Middle English lexis from both these points of view, beginning with semantic changes and then examining collocational studies and investigating how we understand Middle English vocabulary within the context of the evidence provided by historical lexicography. The second part of the chapter is concerned with the structure of the Middle English lexicon, examining word formation and then briefly considering word borrowing and its effects on the structures of word fields in Middle English.