In Washington, security has been stepped up at hotels, embassies and meeting sites of the nuclear security summit. With 47 governments attending, it's thought to be the biggest gathering of world leaders ever in the nation's capital.
Today, President Obama met one-on-one with leaders from several nations, including India and Pakistan; both have nuclear weapons.
Top administration officials appeared on news talk shows to promote the summit's focus on securing nuclear materials - the so-called "loose nukes."
By one estimate there's enough weapons usable nuclear material to build more than 120,000 nuclear bombs.
The big fear is that terrorists could get their hands on some of it. The threat is examined in a new documentary about nuclear weapons, "Nuclear Tipping Point."
"Terrorist groups like al Qaeda - but not limited to al Qaeda - are seeking this nuclear material and would like to make a weapon," former senator and nuclear security expert Sam Nunn says in the documentary. "They've said so and I believe them."
Not invited to the summit are countries perceived as dangerous or uncooperative such as Iran which is trying to build a nuclear weapon, and North Korea, which has nuclear warheads.
But, "This administration can't address the Iran and North Korea threats alone," said Corey Hinderstein of the Nuclear Threat Initiative. "They have to build an international consensus."
Hinderstein says that's just what the two-day summit will help do.
Analysts say president Obama is not on track to fulfill his campaign pledge of securing all loose nuclear material by the end of his first term, but this summit is a key milestone along the way.