The 1958 US–UK Mutual Defense Agreement, or UK–US Mutual Defence Agreement, is a bilateral treaty between the United States and the United Kingdom on nuclear weapons co-operation. The treaty's full name is Agreement between the Government of the United States of America and the Government of the United Kingdom of Great Britain and Northern Ireland for Cooperation on the uses of Atomic Energy for Mutual Defense Purposes. It allows the US and the UK to exchange nuclear materials, technology and information. The US has nuclear co-operation agreements with other countries, including France and other NATO countries, but this agreement is by far the most comprehensive. Because of the agreement's strategic value to Britain, Harold Macmillan (the Prime Minister who presided over the United Kingdom's entry into the agreement) called it "the Great Prize".