Full-body scanners, also known as "naked scanners", are used in airports and other government facilities to detect metallic and nonmetallic objects hidden beneath people's clothes. In many countries, they play a critical part in airline security, but they have also been criticized for being unsafe, ineffective, and an invasion of privacy. To shed scientific lights on these questions, we conducted the first rigorous, independent security evaluation of such a system. We bought a government-surplus Rapiscan Secure 1000 full-body scanner on eBay and extensively tested it in our lab. We found that it's possible to conceal knives, guns, and explosives from detection by exploiting properties of the device's backscatter X-ray technology. We also investigated computer security threats: malicious software and hardware that can compromise the effectiveness, safety, and privacy of the machine. In this talk, we'll explain how full-body scanners work, describe the results of our experiments, and draw lessons to inform transportation security, embedded systems security, and the public debate over secretive and privacy invasive government technologies.