William Brereton, 3rd Baron Brereton FRS (4 May 1631 – 17 March 1680) was an English mathematician and politician who sat in the House of Commons in 1659 and became Baron Brereton in the Irish peerage in 1664. He was chairman of the Committee of Accounts, better known as the Brooke House Committee, in 1667–1670. In that capacity he clashed repeatedly with Samuel Pepys, whose description of Brereton in his Second Diary, or Brooke House Journal, although no doubt biased, is the best portrait we have of the man.[1]