Every day thousands of people worldwide consult Roget's Thesaurus. How many stop to consider why that endlessly useful reference book is so called? Of those who know that it owes its name to the man who first devised it, how many know anything more about him? Yet Peter Mark Roget was one of the most remarkable men of the nineteenth century and he achieved much in his long life. He did not even begin the great work of classification which bears his name until he was 70. Before that, the polymathic Roget had already made his own contributions to knowledge in a dozen different fields from optics and anatomy to mathematics and education. He would probably have been surprised that his posthumous reputation rests on his thesaurus. No doubt he would have expected that it would be his involvement in the foundation of the University of London that would be his lasting legacy. Or his books on magnetism, galvanism and physiology. Or his scientific papers on persistence of vision, with their later impact on the development of motion pictures. Or his association with major thinkers such as the computer pioneer Charles Babbage and the philosopher Jeremy Bentham. The range of his interests was astonishing and, for sixty years, he was at the centre of the intellectual revolution of his times. Nick Rennison's biography reveals the full story of Roget's involvement with the great issues and the great personalities of the nineteenth century and recounts the forgotten life behind one of the most famous of all reference books.