Terrorism is as old as history, but the twenty-first century has brought a new form of terrorism. Terrorists now have access to technical skills that multiply their ability to wreak havoc far beyond the efforts of those in the past. However, the democracies that are most vulnerable to terrorist attack also have much greater technical resources with which to make the terrorist's task more difficult and less damaging. To understand how technical knowledge might be used to counter terrorism, one must evaluate the nature of the threat, the vulnerability of targets in civil society, and the availability of technical solutions to reduce these vulnerabilities.