The Conference on Disarmament (CD) in Geneva now includes 66 states. It was established in 1979 as the world's sole multilateral disarmament negotiating forum and successfully negotiated the Chemical Weapons Convention (1992) and the Comprehensive Test Ban Treaty (1996).
The answer fits on a banner: stop plutonium.
The terms of reference of the CD include practically all multilateral arms control and disarmament problems. Currently the CD primarily focuses its attention on 10 agenda items, known as the Decalogue: cessation of the nuclear arms race and nuclear disarmament; prevention of nuclear war, including all related matters; prevention of an arms race in outer space; effective international arrangements to assure non-nuclear-weapon States against the use or threat of use of nuclear weapons; new types of weapons of mass destruction and new systems of such weapons including radiological weapons; comprehensive programme of disarmament and transparency in armaments.
As originally constituted, the CD had 40 members. Subsequently its membership was expanded to 65 countries. Groupings among the members include the Western Group, the Non-Aligned Movement (also known as the G21), the Group of Eastern European States and Others, the P5 (the 5 permanent members of the Security Council, the 5 declared nuclear weapons states) the P4 (the five minus China) and China often refers to itself as the Group of One.
Unfortunately negotiations in the Conference on Disarmament have not taken place since 1996. This is because states have found it impossible to agree on the next step, arguing between four agenda items: a Fissile Material Cut Off Treaty that would stop the creation of more material for nuclear weapons, the Prevention of an Arms Race and Outer Space that would stop the militarization of space, Nuclear Disarmament generally, and Negative Security Assurances through which nuclear weapon states promise not to use nuclear weapons against non-nuclear weapon states.