Sir Richard Clarke, known always as Otto, was a highly unusual and controversial Treasury civil servant responsible for a wide range of new initiatives in Whitehall between 1945 and 1970. A Cambridge Wrangler, a pre-war chess champion and originator of the Financial Times Index he entered Whitehall in war time more literate and numerate than most of his contemporaries. He regarded Keynes with whom he worked as his model. He was one of a new breed of post-war forceful civil servants, and one of the few capable of original thought. This biographical article tries to assess Otto's contribution, especially in the period 1945-52, compared with that of his contemporaries, in working out the new economic and financial strategy needed for post-war recovery. It also glances at his later career as the main architect of the P.E.S.C. system for controlling public expenditure.