- 1947: Seven minutes to midnight

The clock first appears as a symbol of nuclear danger.

- 1949: Three minutes to midnight

The Soviet Union explodes its first atomic bomb.

- 1953: Two minutes to midnight

The United States and the Soviet Union test thermonuclear devices within nine months of one another.

- 1960: Seven minutes to midnight

Growing public understanding that nuclear weapons made war between the major powers irrational amid greater international scientific cooperation and efforts to aid poor nations.

- 1963: Twelve minutes to midnight

The US and Soviet signing of the Partial Test Ban Treaty "provides the first tangible confirmation of what has been the Bulletin's conviction in recent years -- that a new cohesive force has entered the interplay of forces shaping the fate of mankind."

- 1968: Seven minutes to midnight

France and China acquire nuclear weapons; wars rage in the Middle East, the Indian subcontinent, and Vietnam; world military spending increases while development funds shrink.

- 1969: Ten minutes to midnight

The US Senate ratifies the Nuclear Non-Proliferation Treaty.

- 1972: Twelve minutes to midnight

The United States and the Soviet Union sign the first Strategic Arms Limitation Treaty and the Anti-Ballistic Missile Treaty.

- 1974: Nine minutes to midnight

SALT talks reach an impasse; India develops a nuclear weapon.

- 1980: Seven minutes to midnight

The deadlock in US-Soviet arms talks continues; nationalistic wars and terrorist actions increase; the gulf between rich and poor nations grows wider.

- 1981: Four minutes to midnight

Both superpowers develop more weapons for fighting a nuclear war. Terrorist actions, repression of human rights, and conflicts in Afghanistan, Poland and South Africa add to world tension.

- 1984: Three minutes to midnight

The arms race accelerates.

- 1988: Six minutes to midnight

The United States and the Soviet Union sign a treaty to eliminate intermediate-range nuclear forces; superpower relations improve; more nations actively oppose nuclear weapons.

- 1990: Ten minutes to midnight

The Cold War ends as the Iron Curtain falls.

- 1991: Seventeen minutes to midnight

The United States and the Soviet Union sign the long-stalled Strategic Arms Reduction Treaty (START) and announce further unilateral cuts in tactical and strategic nuclear weapons.

- 1995: Fourteen minutes to midnight

Further arms reductions stall while global military spending continues at Cold War levels. Risks of nuclear "leakage" from poorly guarded former Soviet facilities increase.

- 1998: Nine minutes to midnight

India and Pakistan "go public" with nuclear tests. The United States and Russia cannot agree on further deep reductions in their nuclear stockpiles.

- 2002: Seven minutes to midnight

The United States rejects a series of arms control treaties and announces it will withdraw from the Anti-Ballistic Missile Treaty. Terrorists seek to acquire and use nuclear and biological weapons.

- 2007: Five minutes to midnight.

North Korea's recent test of a nuclear weapon, Iran's nuclear ambitions, a renewed emphasis on the military utility of nuclear weapons," and the continued presence of 26,000 US and Russian nuclear weapons are cited.