The great American cryptologist William F. Friedman and two colleagues visited Bletchley Park and other British code-breaking facilities in the spring of 1943. Friedman kept a diary while in the UK that gives a meticulous account of his activities during the mission. Friedman's diary is the subject of this article, which originally appeared in the journal Intelligence and National Security. Closely read, the diary sheds light on the role Friedman and his colleagues played in negotiating the 1943 intelligence-sharing agreement between the United States and Great Britain and helps resolve problems with the dating of that agreement. It gives us Friedman's informal assessment of the British war-time codebreaking effort. And it presents a portrait of Friedman the man as do none of his other writings.