By Jonathan Marcus
Diplomatic correspondent, BBC News
The summit is to be held at the Washington Convention Center
Fresh from his success in signing a new strategic arms reduction treaty with the Russians in Prague, US President Barack Obama is hosting a nuclear security summit in Washington DC.
With some 47 countries in attendance it will be one of the largest gatherings of its kind in the US capital since the late 1940s.
This will be the third element in a nuclear season that began with this month's unveiling of the Obama administration's nuclear strategy, the Nuclear Posture Review.
This identified nuclear proliferation - the spread of nuclear weapons and the danger that they might fall into the hands of terrorist groups - as now the key nuclear threat to America's security.
That was step one. Step two was the meeting between Mr Obama and his Russian counterpart, Dmitry Medvedev, in Prague that got the strategic arms reduction process back on track. Step three will be this week's Nuclear Security Summit in Washington.
All three events are aimed at strengthening Mr Obama's hand as he heads into step four: the review of the Nuclear Non-Proliferation Treaty (NPT) scheduled for next month in New York.
The NPT is the centre-piece of international efforts to halt the spread of nuclear weapons. However the treaty is looking increasingly threadbare.
Pressure has grown for a new treaty on nuclear weapons
Both North Korea and Iran have driven a coach and horses through its provisions.
Pyongyang has abandoned the agreement altogether and is widely thought to have built a small nuclear weapons arsenal.
Tehran remains within the treaty but refuses to obey UN Security Council calls to halt its enrichment activities, amidst concerns that Iran's ambition - despite its vehement denials - is also to have the bomb.
But the NPT regime has other problems too.
The five declared nuclear weapons states - the US, France, China, Britain and Russia - agreed both to assist with the spread of peaceful nuclear technology but also, ultimately, to get rid of their own atomic weapons.
Many countries around the world believe that they have not moved far enough or fast enough.
Mr Obama wants to build support to shore up the NPT regime - hence his renewed emphasis upon arms control - but he also wants to reinforce it with a whole range of additional measures, of which this nuclear security summit is an important element.
In the past, both Pakistan and North Korea have been willing to export nuclear technology or know-how.
The crucial problem for a would-be nuclear weapons state is to get the nuclear material it needs for a bomb.
One path is to develop a complex and expensive uranium enrichment programme - the reason why so much suspicion has fallen upon Tehran. The other is to actually buy in or steal the fissile material needed.
'Dirty bomb' fears
The threat here is not only from governments with a desire to own nuclear bombs or nuclear-tipped missiles. A far more pressing concern comes from the potential nuclear ambitions of non-state actors or terrorist groups.
Their goal may be to obtain a small nuclear device but equally they may just want to get hold of radioactive material to build a so-called "dirty bomb".
This uses conventional explosives to spread radioactive material over a wide-area.
Such a "dirty bomb" could be used to contaminate key parts of a major city, leading to huge disruption, economic chaos and long-term health problems.
So the goal of this summit is to batten down the hatches on nuclear materials - especially the fissile materials that might be used in bomb-making, plutonium and highly-enriched uranium - but also the more widespread sources of radioactive substances that could be used for a "dirty bomb".
President Obama's goal is to obtain agreement upon a plan to secure all such vulnerable nuclear material within four years. Much will depend upon the detail.
Will this be more than just a well-intentioned summit communique? Will there be clear targets and will nations meet again to review progress before the four-year period is up ?
Inevitably this extraordinary summit will be overshadowed by politics. Chinese President Hu Jintao is attending - a plus for Sino-US relations. Israel's Prime Minister Benjamin Netanyahu will not attend.
No formal reason has been given though there are suggestions that either Mr Netanyahu feared being ambushed by Arab states who might raise concerns about Israel's own nuclear arsenal or that he did not want to face more pressure from the Obama administration to give ground on settlement construction in Jerusalem.
Israel will nonetheless be represented by a senior minister and says it fully backs the summit's agenda.
The presence of Israel, India and Pakistan at this summit is fascinating.
All three are believed to have nuclear weapons and none of them have signed the NPT.
Israel's arsenal clearly has wider ramifications in the Middle East. India and Pakistan's nuclear rivalry is seen by experts as a serious concern given the huge conventional military imbalance between them. And Pakistan is also a major worry in terms of the security of its nuclear installations and materials.
Having all three on board is an attempt by the Obama administration to extend the circle of nuclear security in new directions.
This is an unprecedented gathering. Mr Obama will hope for an unprecedented outcome.