On 6 August 1945, the B-29 bomber Enola Gay dropped an atomic bomb on Hiroshima. The United States would drop a second bomb on another Japanese city, Nagasaki, three days later. At that time, those were the only two functioning nuclear weapons in the world.
Today, nine countries hold a total of 14,900 warheads, according to the International Campaign to Abolish Nuclear Weapons. ICAN is the winner of the 2017 Nobel Peace Prize for "its work to draw attention to the catastrophic humanitarian consequences of any use of nuclear weapons".
Whilst there are nine countries that possess nuclear weapons, a further five house the warheads, and 23 others are part of nuclear alliances.
Since the end of the Cold War in 1991, various treaties have lowered the number of nuclear warheads, as shown in this timeline.