How are words represented in the mind and woven into sentences? How do children learn how to use words? Currently there is a tremendous resurgence of interest in lexical semantics. Word meanings have become increasingly important in linguistic theories because syntactic constructions are sensitive to the words they contain. In computational linguistics, new techniques are being applied to analyze words in texts, and machine-readable dictionaries are being used to build lexicons for natural language systems. These technologies provide large amounts of data and powerful data-analysis techniques to theoretical linguists, who can repay the favor to computer science by describing how one efficient lexical system, the human mind, represents word meanings. Lexical semantics provides crucial evidence to psychologists, too, about the innate stuff out of which concepts are made. Finally, it has become central to the study of child language acquisition. Infants are not born knowing a language, but they do have some understanding of the conceptual world that their parents describe in their speech. Since concepts are intimately tied to word meanings, knowledge of semantics might help children break into the rest of the language system. Lexical and Conceptual Semantics offers views from a variety of disciplines of these sophisticated new approaches to understanding the mental dictionary.