Making aviation security more cost-effective is hampered by a lack of understanding of the costs and benefits of security interventions. Moreover, there will always be considerable uncertainty about terrorists{u2019} capabilities and decisionmaking, security system performance, and the tangible and intangible costs of security measures. This volume focuses on exploring ways to use cost-benefit and other types of analysis to improve aviation security decisionmaking in spite of such uncertainties. The authors present a set of analyses that discuss how historical data on aviation security can inform security planning; examine ways to address uncertainty about the costs of security measures; discuss the ways in which different layers of a security system interact; offer a method for incorporating deterrence into the assessment of security measures via the concept of a risk-reduction threshold, using the Federal Air Marshal Service as an example; examine tradeoffs between intended and unintended consequences of security measures, using a trusted traveler program as an example; and discuss the merits of high- versus low-resolution models of aviation terrorism for informing policy. These analyses contribute to filling some of the current gaps in the assessment of the costs, benefits, and efficiency of aviation security measures and strategies.