Marquess of Lansdowne is a title in the Peerage of Great Britain created in 1784, and held by the head of the Petty-Fitzmaurice family. The first Marquess served as Prime Minister of Great Britain.

Sir William Petty was an English economist, physician, scientist and philosopher. He first became prominent serving Oliver Cromwell and the Commonwealth in Ireland. He developed efficient methods to survey the land that was to be confiscated and given to Cromwell's soldiers. He also remained a significant figure under King Charles II and King James II, as did many others who had served Cromwell.