Among the Clarendon papers held by the Bodleian Library and the Portland Manuscripts appear copies of a long and detailed letter of advice written to Charles II on the eve of the Restoration. The “Advice” was atributed to Edwad Hyde, Earl of Clarendon and Lord Chancellor during the early years of Charles’s reign. In 1903, however, Arthur Strong found that the handwriting of the Welbeck copy was identifical to other documents written by William Cavendish, Earl of Newcastle. Other evidence corrobates Strong’s claim. The letter was apparently written by Newcastle in late 1658 or early 1659 and presented to Charles during the spring of 1659. Here is the text of Newcastle’s letter and and an Introduction by Thomas Slaughter, who transcribed the letter.