This book explores the far-reaching implications for linguistic theory of Eleanor Rosch's seminal work on categorization. A common assumption has been that all members of a category necessarily share a common set of attributes. Rosch's work suggests, instead, that categories are definable in the first instance in terms of best examples, or 'prototypes', and that things get associated with the category on the basis of some kind of similarity with the prototype. The book articulates a number of assumptions and constructs which have become known, especially through the work of George Lakoff and Ronald Langacker, as Cognitive Linguistics. It contributes to a better understanding of this important trend in linguistic research, and will be of interest not only to linguists but also to students of neighbouring disciplines. For this second edition the author has written an additional chapter which surveys the latest developments in the field, concentrating particularly on lexical semantics.