The attempted bombing of Northwest flight 253 in December 2009 underscores the need for effective aviation security programs. Aviation security remains a daunting challenge with hundreds of airports, thousands of aircraft, and thousands of flights daily carrying millions of passengers and pieces of checked baggage. The Department of Homeland Security's (DHS) Transportation Security Administration (TSA) has spent billions of dollars and implemented a wide range of aviation security initiatives. Three key layers of aviation security are (1) TSA's Screening of Passengers by Observation Techniques (SPOT) program designed to identify persons who may pose a security risk; (2) airport perimeter and access controls security; and (3) checked baggage screening systems. This testimony provides information on the extent to which TSA has taken actions to validate the scientific basis of SPOT, strengthen airport perimeter security and access controls, and deploy more effective checked baggage screening systems. This statement is based on prior reports GAO issued from September 2009 through July 2011 and selected updates in June and July 2011. GAO analyzed documents on TSA's progress in strengthening aviation security, among other things.