International flights bound for the United States continue to be targets of terrorist activity, as demonstrated by the October 2010 discovery of explosive devices in air cargo packages bound for the United States from Yemen. The Transportation Security Administration (TSA) is responsible for securing the nation's civil aviation system, which includes ensuring the security of U.S.-bound flights. As requested, GAO evaluated (1) the steps TSA has taken to enhance its foreign airport assessment program since 2007, and any remaining program challenges; (2) TSA's assessment results, including how TSA uses the results to guide future efforts; and (3) what opportunities, if any, exist to enhance the program. To conduct this work, GAO reviewed foreign airport assessment procedures and results, interviewed TSA and foreign aviation security officials, and observed TSA conduct a foreign airport assessment. While these interviews and observations are not generalizable, they provided insights on TSA's program. This is the public version of a sensitive report GAO issued in September, 2011. Information that TSA deemed sensitive has been omitted.Since 2007, TSA has taken a number of steps to enhance its foreign airport assessment program, some of which were taken in response to GAO's prior recommendations. However, challenges remain in gaining access to some foreign airports, developing an automated database to better manage program information, prioritizing and providing training and technical assistance to foreign countries, and expanding the scope of TSA's airport assessments to include all-cargo operations. TSA has various efforts under way to address these challenges. Based on GAO's analysis of TSA's foreign airport assessments conducted from fiscal year 2006 through May 2011, some foreign airports complied with all of TSA's aviation security assessment standards; however, TSA has identified serious noncompliance issues at a number of foreign airports. Common areas of noncompliance included weaknesses in airport access controls and passenger and baggage screening. Moreover, GAO's analysis showed variation in airport compliance across geographic regions and individual security standards, among other things. However, TSA has not yet taken steps to evaluate its assessment results to identify regional and other trends over time. Developing a mechanism to evaluate its assessment results could help support TSA's priorities for aviation security training and technical assistance, inform its risk management decision making by identifying any trends and security gaps, and target capacity building efforts. Opportunities also exist for TSA to make additional program improvements in several key areas. Providing TSA decision makers with more specific criteria and definitions could provide greater assurance that such determinations are consistent across airports over time. In addition, there are opportunities for TSA to increase program efficiency and effectiveness by, for example, conducting more targeted foreign airport assessments and systematically compiling and analyzing security best practices. Taking such actions could help TSA better focus its assessments to address areas of highest risk, and identify security best practices and technologies that may be applicable to enhancing the security of both foreign and domestic airports. GAO recommends that TSA develop a mechanism to evaluate its assessment results to identify any trends, and target resources and future activities; establish criteria for determining foreign airport vulnerability ratings; and consider the feasibility of conducting more targeted assessments and compiling information on aviation security best practices. DHS agreed with the recommendations.