The title Baron Greystock (or Greystoke) has been created twice in the Peerage of England. It was first created when John de Greystock was summoned to parliament in 1295 and it became extinct on his death. It was next created for Sir Ralph de Greystock in 1321, and went into abeyance in 1569, after it had passed into the Dacre family through the marriage of Thomas Dacre, 2nd Baron Dacre to Elizabeth Dacre, 6th Baroness Greystoke.
The title Lord Fitzwilliam seems to have been associated with the second creation, but this matter is unclear.
Representatives of the 4th Baron Dacre and 8th Baron Greystock: