This monograph provides a practical definition of terrorism risk, presents a method of estimating it, and demonstrates a framework for evaluating this method. Results support conclusions on how to improve risk-based resource allocation. The Department of Homeland Security is responsible for protecting the United States from terrorism. It does so partly through the Urban Areas Security Initiative, though its distribution has been criticized for not reflecting risk. This monograph offers a practical definition of terrorism risk and a method for estimating it that addresses inherent uncertainties. It also demonstrates a framework for evaluating alternative risk estimates. Finally, it makes five recommendations for improving resource allocation.