For more than 30 years, probabilistic risk analysis (PRA) has been a major tool for assessing risks and informing risk management decisions by government and businesses in areas as diverse as industrial safety, environmental protection, and medical decision making. The more recent application of PRA to terrorism risk is new, however, and not uncontroversial, as evidenced by the commentary by Parnell and colleagues in the December 2008 issue of "Biosecurity and Bioterrorism." The National Research Council on Methodological Improvements to the Department of Homeland Security's Biological Agent Risk Analysis has argued that because of the adaptive nature of the terrorist adversary, alternative tools like decision trees, game theory, and agent-based modeling are needed to assess the risks of terrorist events and that probabilistic risk analysis is not valuable. In this commentary, we take a broad view of PRA, including any probabilistic approach involving tools like event trees, fault trees, decision trees, and influence diagrams. We make 2 points: (1) PRA is useful to quantify terrorism risk, and (2) event trees can help to decompose the universe of terrorism scenarios.