The United States has sought to develop and deploy ballistic missile defenses for more than 50 years. Since President Reagan's Strategic Defense Initiative (SDI) in FY1985, Congress has provided about $110 billion for ballistic missile defense programs and studies. National missile defense (NMD) has proven to be challenging and deployment of an effective NMD system remains uncertain. Until recent years, NMD was a divisive political and national security issue. Debate has focused on the nature and immediacy of foreign missile threats to the United States and its interests, the pace and adequacy of technological development, the foreign affairs and budgetary costs of pursuing missile defenses, and implications for deterrence and global stability. In the mid-1980s and into the early 1990s, Congress reacted to these concerns and questions by reducing requested missile defense budgets and providing legislative language to guide the development of missile defense programs and policy. During this time, many in Congress appeared more concerned than the Defense Department and the military about near-term threats to forward-deployed U.S. military forces posed by shorter range ballistic missiles. Congress demonstrated those concerns by supporting the development and deployment of theater missile defenses (TMD), oftentimes over the objections of the Defense Department. Since the end of the 1991 Persian Gulf War, and especially over the past several years, Congress generally has supported larger missile defense budgets. For FY2007, Congress appropriated $9.3 billion for missile defense programs, making it the largest Defense Department acquisition program. The primary technological concept for missile defense since the early 1980s has been 'hit-to-kill' interceptor missiles. The utility of hit-to-kill as a ballistic missile defense (BMD) concept over the past 25 years remains mixed. Alternatives to the hit-to-kill concept have also been pursued. One is the development of laser technology and the platforms on which lasers might be based. For most missile defense advocates the Airborne Laser (ABL) program represents the most promising near-term effort. Although the Air Force contends that the ABL is mature technology, some observers have questioned whether this technical assessment is accurate, pointing out that the various ABL components have yet to be fully integrated and tested. Considerable debate also continues over whether the ABL will be capable of dealing with likely future ballistic missile threats. The effort that led to the ABL dates to the early 1970s when the Air Force began development of an Airborne Laser Laboratory (ALL)—a modified KC-135A aircraft—to demonstrate that a high-powered laser mounted on an aircraft platform could destroy an attacking missile. After 10 years of research, development, and field testing (culminating in 1983) the ALL program announced that lasers had managed to "destroy or defeat" five Sidewinder air-to-air missiles and a simulated cruise missile at short range. The ALL aircraft was retired in 1984 because its research purpose was considered no longer necessary. Although the ALL test targets were not ballistic missiles, the Air Force and the Defense Department became increasingly interested in the possibility of using high-powered lasers aboard aircraft to destroy enemy ballistic missiles during their boost phase. Through the 1980s and mid-1990s, further research on various ground laser concepts and designs and tracking and beam compensation tests convinced Pentagon officials to proceed with the conditional development of the ABL in June 1998 (although low-level funding for the program began as early as FY1994). Currently, the ABL program is the primary focus of the Missile Defense Agency's (MDA) Boost Defense program. The ABL's lethality test demonstration, which is designed to test the various subsystems and target and destroy a ballistic missile, has been delayed many times. According to the Missile Defense Agency, that lethality test, which was initially scheduled for late FY2003, is now planned for August 2009. Congress has provided strong funding support ballistic missile defenses in the face of growing concerns about the proliferation of missiles around the world. Of all the current efforts, most missile defense advocates believe the ABL shows the best near-term promise for destroying enemy ballistic missiles during their boost-phase. While the missile is still in the earth's atmosphere, the airborne laser would seek to rupture or damage the target's booster skin to cause the missile to lose thrust or flight control and fall short of the intended target before decoys, warheads, or submunitions are deployed. The expectation is that this would occur near or even over the enemy's own territory. Second, although the United States has primarily pursued kinetic energy kill mechanisms for missile defense some 25 years, many defense analysts believe that if the United States chooses to pursue increasingly effective missile defenses for the longer term future, then alternative concepts such as high-powered lasers may be the answer. This report tracks the current program and budget status of the Airborne Laser program. In addition, this report examines several related issues that have been of interest to Congress. It will be updated occasionally as necessary. This report does not provide a technical overview or detailed assessment of the ABL or Air-Based Boost Program. It is envisioned that the ABL would use a high-power chemical laser mounted in the aft section of a modified Boeing 747 aircraft to destroy or disable all classes of ballistic missiles during the initial portion or first several minutes of their flight trajectory (from shortly after launch and before they leave the earth's atmosphere). Analysts indicate that during this period (up to several minutes) the missile is at its most vulnerable stage—it is slower relative to the rest of its flight, it is easier to track because the missile is burning its fuel and thus has a very strong thermal signature, and it is a much larger target because any warhead has not yet separated from the missile itself. Analysts also point out the advantages of destroying the missile before any warhead, decoys, or submunitions are deployed, and potentially over the enemy's own territory. The ABL program will integrate a weapons-class chemical laser aboard a modified Boeing 747-400 series freighter aircraft (747-400F). The chemical laser has been assembled in the System Integration Laboratory (SIL), a 747-200 fuselage, and tested. The Air Force acquired the 747-400F in January 2000 directly from the Boeing Commercial Aircraft assembly line and flew it to Wichita, Kansas, where Boeing workers made extensive modifications to the aircraft. Among other things, they grafted huge sheets of titanium to the plane's underbelly for protection against the heat of the laser exhaust system, and added a 12,000-pound bulbous turret on the plane's front to house the 1.5 meter telescope through which the laser beams would be fired. This plane made its maiden flight in July 2002; it logged 13 more flights in 2002 before relocating to Edwards Air Force Base in California. Since 2002, the focus of the ABL program has been on system integration, an effort that is considered challenging. Officials have reported completing ground integration and testing of the Beam Control Fire Control (BCFC) segment and most of that segment's integration into the ABL aircraft. Additionally, six laser modules in the SIL have been integrated and tested. Further integration and testing of the BCFC, laser modules in the SIL, and communications links took place in 2005. However, the primary goal to have achieved a lethality test by 2005 was not met. That test has now been moved further to August 2009. Program officials said the delay was due largely to program restructuring and budget changes. Others suggested that technical and integration problems continue to prove more challenging than anticipated. In 2006, Boeing announced successful surrogate low-power laser testing from the ABL aircraft. In October 2006, Boeing rolled out the ABL aircraft in Wichita, Kansas, announcing successful completion of major system integration milestones in preparation for some flight testing that will lead to the lethality test in August 2009. As of January 2007, ABL had completed over 50 flight tests. In March 2007, the ABL successfully completed the first in a series of in-flight tracking laser firings at an airborne target. Officials argue this is an important step toward demonstrating the aircraft's ability to engage an airborne target. Major ABL subsystems include the lethal laser, a tracking system, and an adaptive optics system. The kill mechanism or lethal laser system (as distinct from the other on-board acquisition and tracking lasers) is known as COIL (Chemical Oxygen Iodine Laser). COIL generates its energy through an onboard chemical reaction of oxygen and iodine molecules. Because this laser energy propagates in the infrared spectrum, its wavelength travels relatively easily through the atmosphere. The acquisition, tracking, and pointing system (also composed of lasers) helps the laser focus on the target with sufficient energy to destroy the missile. As the laser travels to its target, it encounters atmospheric effects that distort the beam and cause it to lose its focus. The adaptive optics system compensates for this distortion so that the lethal laser can hit and destroy its target with a focused energy beam. The current ABL program began in November 1996 when the Air Force awarded a $1.1 billion PDRR contract (Program Definition Risk Reduction phase) to several aerospace companies. The contractor team consists of Boeing, Lockheed Martin, and Northrop Grumman (formerly TRW). Boeing Integrated Defense Systems (Seattle, WA) has overall responsibility for program management and systems integration, development of the ABL battle management system, modification of the 747 aircraft, and the design and development of ground-support subsystems. Lockheed Martin Space Systems (Sunnyvale, CA) is responsible for the design, development, and production of ABL target acquisition, and beam control and fire control systems. Northrop Grumman Space Technology (Redondo Beach, CA) is responsible for the design, development, and production of the ABL high-energy laser. A number of subcontractors are also involved.