Technical countermeasures are key components of national efforts to combat terrorist violence. Efforts to collect data about and disrupt terrorist activities through human intelligence and direct action, information gathering, and protective technologies complement technical countermeasures, helping to ensure that terrorists are identified, their ability to plan and stage attacks is limited, and, if those attacks occur, their impact is contained. Given the potential effect of such measures on the terrorists' capabilities, it is not surprising that they act to reduce or neutralize the impact of defensive technologies on their activities. In the event that the terrorists counter-efforts are successful, the value and protection provided by defensive technologies can be substantially reduced. Through case studies of terrorist struggles in a number of nations, this document analyzes the nature and impact of such terrorist counter-efforts on the value of defensive technologies deployed against them. The information presented here should be of interest to homeland security policy makers in that it identifies potential weaknesses in defensive technology systems, thereby informing threat assessment and providing a basis for improving the design of future defensive technologies. It extends the RAND Corporation's ongoing research on terrorism and domestic security issues.