This is the Government Accountability Office's (GAO) second report reviewing the Joint Strike Fighter (JSF) program. GAO also assessed the program's acquisition approach---in terms of capturing knowledge for key investment decisions---and identified an alternative to improve outcomes. JSF, the Department of Defense's (DOD) most expensive aircraft program, represents 90 percent of the remaining planned investment for recapitalizing DOD's aging tactical aircraft fleet. GAO recommends Congress should consider delaying authorizations and appropriations for JSF procurement until a new business case is developed and flight testing demonstrates the design and integrated mission systems work. GAO included this matter for consideration because DOD did not plan to make changes as a result of recommendations. GAO recommends that DOD delay investing in production until flight testing shows that the JSF performs as expected, and that the program develop a plan, consistent with DOD's preferred policy, to adopt an evolutionary approach that limits new content for each increment to proven technologies and design. DOD partially concurred, but believes that its current practices achieve the GAO recommendations' objectives.