Authorities often point to Russia, the former Soviet republics and Pakistan as the countries that pose the biggest risk. The former USSR held a significant stock of nuclear material, which was put at risk during the political upheaval in former Soviet countries that followed the fall of communism.
Pakistan also has a significant cache of nuclear material, following its development of atomic weapons in an arms race with India in the 1970s. Pakistani scientist Abdul Qadeer Khan, widely viewed as the father of Pakistan's nuclear program, has long been accused of sharing nuclear secrets with rogue states such as North Korea. He is currently banned from leaving Pakistan, after being placed under house arrest there for several years.
Recent incidents suggest that relaxed security at nuclear facilities worldwide remains a serious and pressing issue. This includes the United States, which mistakenly shipped nuclear triggers to Taiwan in 2006.
According to the Kennedy School's Belfer Center, just two months ago, peace activists breached the Kleine-Borgel air base in Belgium, a location where nuclear weapons are believed to be stored. The activists climbed in undetected over a simple chain-link fence and were on the property for 90 minutes before they were arrested.
That incident came after a similar breach at the air base in 2009 and the 2001 arrest of a al Qaeda operative who was planning to bomb the same facility. The group had bought photographs of the base from an insider.
Nuclear facilities in the United States are protected by armed guards, concrete barriers, sophisticated cameras and motion sensors, and the Obama administration wants to make such security precautions standard protocol around the world.
That goal will be difficult, though, because nuclear material is not just restricted to military installations. Components for radiological or dirty bombs can be found in thousands of places around the world, in places as innocuous as hospitals and construction sites.