The U.S. effort to develop and deploy ballistic missile defenses (BMD) based on the concept of hit-to-kill or kinetic energy kill began three decades ago. This effort gained momentum as the primary focus of the U.S. BMD program in the mid-1980s with the announcement of President Reagan's Strategic Defense Initiative (SDI). Since that time, the United States has pursued numerous major kinetic energy BMD programs; these have produced hundreds of various flight test results. These test results and some very limited operational experience in wartime provide sufficient data for at least some conclusions regarding the decades-long U.S. investment in hit-to-kill as a concept for BMD. This overview report examines the U.S. investment in that concept, what that investment has produced, and raises various questions that might be considered. The development of BMD has shown important technological differences between efforts designed to attack and destroy short or medium-range ballistic missiles and those designed for long-range or intercontinental ballistic missiles. Therefore, this report will review and distinguish between the program results of theater missile defense (TMD) and national missile defense (NMD). CRS received historical flight test data from the Missile Defense Agency (MDA) in June 2005. It is important to note that for each of these flight tests there were various primary and multiple secondary objectives. Such flight tests are inherently complex and relatively costly. Therefore, multiple test objectives are designed to maximize the potential benefit derived from each flight test. The determination as to whether each of these objectives was reached was made by each relevant agency or military branch. All of the references to flight test results in this report are derived from the Flight Tests Results memorandum provided by the MDA unless otherwise referenced. CRS currently is awaiting an update of the historical flight test data from MDA, which will be reviewed and included in an updated version of this report later in 2007. Analysis of flight test data shows that the U.S. effort to develop, test, and deploy effective BMD systems based on this concept has produced mixed and ambiguous results. The actual performance in war-time of one kinetic-energy system currently deployed by the United States (i.e., the Patriot PAC-3) is similarly ambiguous. Further, it is not yet possible to assess the operational effectiveness the other deployed system (i.e., the National Defense System) against long-range ballistic missile threats. The United States has pursued four major kinetic energy interceptor long-range BMD or NMD programs since the early 1980s: Homing Overlay Experiment (HOE), Exoatmospheric Reentry Interceptor Subsystem (ERIS), National Missile Defense (NMD), and Ground-based Midcourse Defense (GMD). Each of these is briefly discussed below. The Army developed HOE in the late 1970s and early 1980s to test the viability of the emerging hit-to-kill concept. It conducted four intercept flight tests in 1983 and 1984. Three of the tests failed to intercept the intended target, but the fourth was considered a success. The Army did not identify any secondary flight test objectives. Nonetheless, the nascent SDI program then viewed the single reported success as evidence of the promise of non-nuclear BMD interceptor technologies. The technologies tested in HOE served as the basis for its successor program, ERIS. ERIS went through a lengthy development program before flight testing began in 1991 with the first of four intercept flight tests. Although the first was considered a successful intercept of the target, the following three intercept attempts through 1992 failed to destroy their intended targets. Even so, officials concluded that half of the primary and secondary test flight objectives were accomplished, and that the primary BMD concept being pursued held significant promise. The NMD program followed ERIS with a series of eight flight tests from 1997 to 2001. The first three were planned "fly-by" tests. There were no intercept attempt objectives. The first one failed to launch; however, the other two were deemed successful in their primary objectives. No secondary objectives were identified. Of the five planned intercept attempts, three reportedly intercepted their intended targets; one ended in failure because the interceptor kill vehicle did not deploy and the other failed because the on-board sensors designed to track and intercept the target failed. Officials concluded that 17 of the 20 primary objectives were met or partially met and all the secondary objectives by the planned intercept tests were met. The current GMD program (NMD's successor) began flight testing in 2002. Since that time six flight tests have taken place. Five of these flight tests were planned intercept attempts, with three resulting in failure to intercept. Officials concluded that about 80% of the program's 40 or so primary intercept flight test objectives were met; all the secondary objectives were met fully or partially. In 2004, the GMD undertook a new configuration with a different booster and interceptor. It flew a successful integration flight test (non-intercept test) in early 2004 with all primary and secondary objectives met. This system was deployed in Alaska and California in 2004 and declared operational after eight missiles were placed in silos. Subsequently, two planned intercept flight tests in December 2004 and February 2005 failed to launch. The currently deployed system thus remains to be tested successfully against targets it might be expected to intercept. In September 2006, a successful flight test exercise of the GMD system too place. Although not a primary objective of the data collection test, an intercept of the target warhead was achieved. Flight tests whose primary objectives are intercepts were scheduled for later in 2006, but have been delayed into 2007. Each of the four NMD programs were different, but they built on the limited successes of their predecessors. Of the eighteen or so attempted intercepts since the early 1980s, seven of them were considered successful, or roughly a 39% intercept rate in tests. Officials cited several reasons, including program hardware and software, as well as interceptor silo and target launch failures. From that, there do not appear to be any recognizable patterns that emerge to account for the mostly unsuccessful history of the effort. Nor is there conclusive evidence of a learning curve, such as increased success over time relative to the first tests of the concept 20 years ago. Program supporters can point to limited evidence that, under controlled conditions, there is reason to support the contention that kinetic energy interceptor technology for use against long-range ballistic missiles holds promise. Critics of the flight test effort to date, whether they support missile defense or not in general, can raise questions about the success rate and the realism of the testing effort, given a generation of U.S. investment in its development. Can kinetic energy interceptor technologies for use against long-range ballistic missiles be developed successfully and deployed as an effective part of the U.S. military posture? The answer appears to be ambiguous at this juncture. Can the now-deployed NMD system protect the United States from long-range ballistic missile attacks? Currently, there is insufficient empirical data to support a clear answer. There have been a number of major kinetic-energy TMD programs since the early 1990s: Extended Range Intercept Technology (ERINT), Flexible Lightweight Agile Guided Experiment/Small Radar Homing Intercept Technology (FLAGE/SRHIT), Navy Lightweight Exoatmospheric Projectile (LEAP), the Navy Aegis BMD, Patriot PAC-3, and Theater High Altitude Area Defense (THAAD). Each of these are briefly examined below. The Army's FLAGE/SRHIT program conducted eight flight tests from 1984-1987 to prove the feasibility of lower atmosphere intercepts. Five of these flight tests were planned intercept attempts. From the data provided by MDA all the primary and secondary test objectives in the series were achieved. The targets included stationary targets in the atmosphere and an air-launched target. Only one target, however, was a short-range missile. The degree to which any conclusions might be drawn regarding very short-range hit-to-kill in this effort is therefore limited. Building on the SRHIT effort, the Army's ERINT flight test program (1992-1994) conducted five flight tests. Three of these were planned intercepts; two of these three flight tests successfully intercepted their targets (the failure cited was hardware related). Despite the missed intercept, the Army concluded that all of its primary test objectives for the three tests were met fully or partially, and that all but one of the 26 secondary objectives in the three tests were met. As far as the two non-intercept flight tests were concerned, the Army determined that all of its primary and secondary flight test objectives were met. The Navy developed its own indigenous LEAP program, which flight tested from 1992-1995. Three non-intercept flight tests achieved all primary and secondary objectives. Of the five planned intercept tests, only the second was considered a successful intercept, however. Failures were due to various hardware, software, and launch problems. Even so, the Navy determined that it achieved about 82% of its primary objectives (18 of 22) and all of its secondary objectives in these tests. Building on some of its previous efforts in SRHIT and ERINT, the Army's THAAD program nevertheless experienced significant challenges from 1995 to 1999. After three relatively successful non-intercept flight tests (almost all of the primary and secondary test objectives were partially or fully met), THAAD failed to intercept in seven of its nine planned attempts. However, the THAAD intercept flight test program met about half of its primary and secondary objectives. Because the last two intercepts were successful (the last being in 1999), the Department of Defense and Congress agreed to further develop, but revamp, the THAAD program. The current THAAD program is a redesign of the former THAAD system. Recently, the program conducted its first flight test (non-intercept) to examine the launch, boost, and fly-out functions of the THAAD missile.