Recent events have focused attention on the threat that terrorists with shoulder fired surface-to-air missiles (SAMs) pose to commercial airliners. Most believe that no single solution exists to effectively mitigate this threat. Instead, a menu of options may be considered, including installing infrared (IR) countermeasures on aircraft; modifying flight operations and air traffic control procedures; improving airport and regional security; and strengthening missile non-proliferation efforts. Equipping aircraft with missile countermeasure systems can protect the aircraft even when operating in areas where ground-based security measures are unavailable or infeasible to implement. However, this option has a relatively high cost, between $ 1 million and $3 million per aircraft, and the time needed for implementation does allow for immediate response to the existing terrorist threat. Procedural improvements such as specific flight crew training, altering air traffic procedures to minimize exposure to the threat, and improved security near airports may be less costly than countermeasures and could more immediately help deter domestic terrorist attacks. However, these techniques by themselves cannot completely mitigate the risk of domestic attacks and would not protect U.S. airliners flying to and from foreign airports. On February 5,2003, Rep. Steve Israel and Sen. Barbara Boxer introduced legislation (H.R. 580, 5. 311) calling for the installation of missile defense systems in all turbojet aircraft used in scheduled air carrier service, and other Members of Congress have expressed an interest in holding hearings on this matter. This report will be updated as needed.