Leaders from almost 60 countries are gathered today in Washington, D.C., for an event called the Nuclear Security Summit.
The first one was in 2010.
It was part of an effort by U.S. President Barack Obama to encourage the world to get rid of its nuclear weapons.
Even though the U.S. and Russia signed a treaty in 2010 to reduce their nuclear weapons, and even though an international agreement was reached to slow down Iran's nuclear program, experts say the threat from nuclear weapons is worse than when the first national security summit was held.
Iran and Russia aren't attending this year's summit.
Analysts say there may not be much progress without Russia there.
The U.S. military is modernizing its nuclear arsenal, rather than dismantling it, as President Obama initially had hoped.
Pakistan is rapidly acquiring nuclear material and world leaders are deeply concern about ISIS.
What if the terrorist group gets access to a nuclear weapon?
What if North Korea, a U.S. rival gets one?
It's apparently building up its nuclear program and China and the U.S. are promising to do more to stop the smuggling of nuclear materials.
They drive across the old narrow bridge around 9:00 a.m. each day.
Chinese trucks carrying goods into North Korea.
They leave from Dandong, the border city on the Yalu River.
It's the economic lifeline of North Korea.
China the only country left willing to do significant trade with Kim Jong-Un's regime.
New U.N. sanctions levied against North Korea's nuclear program had impacted that relationship.
For example, North Korean coal exports, important revenue for the country, are now banned if any profits from them might be funneled to sanctioned programs.
China must also now inspect all shipments into and out of the country.
Criticized in the past for not enforcing sanctions, officials deny that, but say they will strongly implement this latest round.
We watched as North Korean trucks drove into China around midday, mostly empty.
They end up in yards like this, loaded up with Chinese goods that get sent back across the border.