The Marine Chronometers at Greenwich is the fifth, and largest, of the distinguished series of catalogues of instruments in the collections of the National Maritime Museum. Housed at the Royal Observatory Greenwich — the 'home of time' and the Prime Meridian of the world — this extraordinary collection, which includes the celebrated marine timekeepers by John Harrison (1693-1776), is generally considered to be the finest of its kind in existence. The book is however much more than just a catalogue, and includes an accessible and engaging history of the chronometer, revealing why these instruments were important in our scientific and cultural history, and explaining, in simple terms, how they worked and were used. A comprehensive Glossary and Bibliography are included to ensure any technicalities are explained and that the reader has suggestions for useful 'further reading'. Over 480 photographs and illustrations, including many fine macro-photographs and line drawings, illustrate the 'jewel-like' beauty of the chronometer's construction and explain the function and subtleties of its mechanism. A chapter on 'How the Chronometer was Made', describes the fine sub-division of labour used to create these special machines, from bare metal, right up to delivery on board ship, and brief biographies of the makers tell the human story behind this important nineteenth-century industry. Another chapter, 'The Evolution of the Chronometer', aimed at collectors, historians and curators, provides clearly structured information on assessing and dating the chronometer, something many find difficult. And, for the dedicated specialist, there is extensive tabulated data on the technical structure of this important collection, a unique resource for future research.