In this Policy Briefing we look at authorities' relative counterterrorism effectiveness focusing on its behavior over time, its impact on preventing casualties and property losses. The study underlying this Policy Briefing used data on transnational terrorism from the ITERATE database (1973-2003) and discrete choice models, to evaluate relative counterterrorism effectiveness while controlling for a variety of terrorists' and authorities' effort attributes. It was found out that the probability of a terrorist incident being stopped by the authorities has increased in the period examined. Furthermore, a negative relationship between authorities' ability to stop an incident and the probabilities of casualties and damages is identified. However, the "ability to stop" exerts higher impact on the probability of property losses compared to casualties.