GAO reviewed the Theater High Altitude Area Defense (THAAD) program to determine whether: (1) planned testing would demonstrate operational effectiveness before a significant number of units are produced for deployment; and (2) missile target resources are adequate to support testing plans. GAO noted that: (1) the current THAAD program review and evaluation provides the Department of Defense (DOD) with the opportunity to: (a) reduce risk and minimize the number of initial quantities of unproven system hardware by reexamining the schedule for operational testing and production; and (b) ensure that realistic targets will be used for testing; (2) the last approved THAAD acquisition plan calls for significant production of deployment hardware almost 2 years before beginning independent operational testing to assess the system’s operational effectiveness; (3) the Army maintains that it needs to buy a number of THAAD systems during low-rate initial production to “ramp-up” to the full rate of production; (4) delaying production until after completing sufficient testing that provides assurance that key performance requirements can be met reduces the risk of buying unproven systems and facilitates production of proven systems at more efficient rates; (5) a suitable target for testing the THAAD system against longer range missiles does not exist, and funds have not been requested for target development and production; and (6) without a longer range test target to represent the more formidable, higher velocity missiles that THAAD could face, the system’s operational effectiveness will remain in doubt and DOD will not have reasonable assurance that it could rely on THAAD in an actual conflict.