Kim Philby was the most notorious British defector and Soviet mole in history. Agent, double agent, traitor and enigma, he betrayed every secret of Allied operations to the Russians in the early years of the Cold War. His two closest friends in the intelligence world, Nicholas Elliott of MI6 and James Jesus Angleton, the CIA intelligence chief, thought they knew Philby better than anyone – and then discovered they had not known him at all. With access to newly released MI5 files and previously unseen family papers, and with the cooperation of former officers of MI6 and the CIA, this definitive biography unlocks what is perhaps the last great secret of the Cold War.