Effective explosives detection involves a combination of materials science, physics and chemistry. But it's more than a scientific problem. Once a technology has been developed, researchers need to know how to implement it at airports, or in the hand of a soldier searching for explosive devices in Iraq. They also need to anticipate emerging threats. David Atkinson says that aviation security checkpoints are effective -- and the problem now is to transfer the technology to shopping malls, buses and subways. And to constantly update it: Explosive threats are changing all the time.