Breakthrough science can lead both to great good and to great evil. The September 11, 2001, terrorist attacks on the World Trade Center and the Pentagon and the anthrax letter attacks that followed highlight the fact that our enemies may use our own advanced science and technology against us. When the dissemination of scientific information might jeopardize national security, the federal government’s primary response has always been to try to control the spread of that information. In a variety of ways, the government has long restricted public access to scientific information in the government’s possession. Since September 11, the government has further tightened access to its own information, withholding from public view not just classified data but also so-called “sensitive” information, the release of which it says could pose a danger to national security.