In this book Werner Hüllen examines Roget's Thesaurus in relation to linguistics, philosophy and history. He explores the influence of Roget's Thesaurus abroad (Germany and the Romance countries). He epitomizes its history and compares the various editions of the book. In lexical case studies he evaluates some entries with pertinence to their cultural and political implications. He discusses the didactic potential of thesauri in general and considers the implications of the Thesaurus for the study of scholarly linguistics and psychology. He discusses how Roget's Thesaurus prepared the way for the more recent idea of network semantics. By analyzing retrieval techniques one can show, he claims, how the words of languages were (and are) stored in the minds of those who speak them. Professor Hüllen concludes by considering the role of synonymy in language from a perspective of cognitive linguistics showing that it is indispensable for communication.