The inspectors dismantled Saddam Hussein's nuclear programme and destroyed stocks of chemical and biological weapons that time. But some weapons are believed to have survived or been rebuilt.
About 100 inspectors, backed by a tough new UN Security Council resolution, now plan to scour Iraq with particle detectors, satellite imagery, ground-penetrating radarand cameras that beam live video back to Vienna.
But most important, experts say, will be knowing where to point these new gadgets.
Inspectors will need a detective's intuition and usable intelligence and tips from Iraqi scientists and defectors.
They will also need to be able to recognise what, for instance, a Scud missile's turbo pump looks like, one official said.
"We can assume Iraqis have moved all sensitive pieces of evidence," said former UN inspector Victor Mizin. "Without some data provided by the Baghdad government, the inspections won't find anything meaningful."
Still, inspectors are bringing in plenty of hi-tech gadgetry all paid for like the entire inspection process by the sale of Iraqi oil.
The International Atomic Energy Agency's 20 nuclear weapons inspectors will check sites using gamma radiation detectors mounted on helicopters or held by hand, said Peter Rickwood, an IAEA spokesman in Vienna.
While the IAEA tracks nuclear items, the UN inspectors will seek banned missile components and the remnants of Hussein's biological arsenal which included anthrax and botulinum toxin and chemical agents sarin, VX and mustard gas.
Further complicating the search, raw materials for the world's most lethal weapons have vital civilian uses in medicines and vaccines.