RAND evaluated a terrorism risk modeling tool developed by the Transportation Security Administration and Boeing to help guide program planning for aviation security. This tool, the Risk Management Analysis Tool, or RMAT, is used by TSA to estimate the terrorism risk-reduction benefits attributable to new and existing security programs, technologies, and procedures. RMAT simulates terrorist behavior and success in attacking vulnerabilities in the domestic commercial air transportation system, drawing on estimates of terrorist resources, capabilities, preferences, decision processes, intelligence collection, and operational planning. It describes how the layers of security protecting the air transportation system are likely to perform when confronted by more than 60 types of attacks, drawing on detailed blast and other physical modeling to understand the damage produced by different weapons and attacks, and calculating expected loss of life and the direct and indirect economic consequences of that damage. This report describes RAND's conclusions about the validity of RMAT for TSA's intended uses and its recommendations for how TSA should perform cost-benefit analyses of its security programs.