There are currently nine nations thought to be in possession of nuclear weapons, and while estimates vary as to the exact number each one has, the Federation of American Scientists (FAS) has provided a ballpark figure.
According to FAS, there are currently about 22,300 nuclear weapons in the world's arsenal, approximately 7,900 of which are operational. Of these, about 2,000 U.S. and Russian warheads are on high alert and ready for use on short notice.
Exact figures are hard to obtain, since weapons information is held close to the vest by each nation for security reasons. However, some information regarding the size and composition of nuclear weapon stockpiles is made public or leaked.
Russia, the United States, France, China, the United Kingdom, Israel, Pakistan, India and North Korea are all thought to possess nuclear weapons, and have been dubbed the "Nuclear Club" by the Council on Foreign Relations.
The United States, Russia, the United Kingdom, France, and China are all considered to be "nuclear weapons states" (NWS) under the Nuclear Non-Proliferation Treaty (NPT) of 1968. The most widely accepted international arms-control agreement, the NPT prohibits the spread of nuclear weapons while supporting the utilization of nuclear energy for peaceful purposes.
North Korea had previously been a part of the NPT, but it withdrew in 2003 while continuing to conduct nuclear tests. The only counties to have never signed the treaty are Israel, India, and Pakistan. India and Pakistan have conducted nuclear tests, while Israel is thought to have nuclear weapons, although it has refused to acknowledge its nuclear arsenal.
An example of the NPT's success in preventing the spread of nuclear weapons and weapons technology is when South Africa, which had previously conducted a nuclear weapons production program, destroyed its nuclear arsenal in order to become a party to the NPT.
It should be noted that thousands of Russian and U.S. nuclear warheads have actually been stored in inactive stockpiles and are waiting to be deployed and processed under the Strategic Offensive Reductions Treaty (SORT). The weapons have not been completely disposed of because their fissile materials can be recycled and used in nuclear reactors.