The Brussels attacks have highlighted the urgent need to make the world's supply of fissile material secure.
Two of the bombers appeared to have previously been monitoring a senior researcher who worked at a Belgian nuclear centre.
The White House reckons that around 2,000 metric tonnes of highly dangerous nuclear material is out there, in both civilian and military programmes around the world.
It is not just used in weapons of war. Highly enriched uranium and separated plutonium are used to generate power, and they are also widely used in medicine.
Early on in his presidency Barack Obama highlighted the risk of nuclear attacks, describing them as "the most immediate and extreme threat to global security".
So he launched a series of Nuclear Security Summits, and the gathering of world leaders in Washington DC this week was the fourth such summit.
On this Inside Story, we ask what international efforts have achieved so far, and what remains to be done?
Presenter: Martine Dennis
Nickolas Roth - Research associate, Belfer Center for Science and International Affairs, Harvard Kennedy School
Hamish de Bretton-Gordon - Chemical weapons specialist, adviser to NGOs in Syria & Iraq
Togzhan Kassenova - Associate, Nuclear Policy Program, Carnegie Endowment for International Peace
Source: Al Jazeera