The findings of a national evaluation of the U.S. Department of Energy＇s low-income Weatherization Assistance Program (＇＇Program＇＇) are used to explain variations in Program effectiveness. Overall, the Program was found to be a cost-effective federal investment. However, local weatherization agencies vary greatly in the effectiveness of their operations, and the energy saved by individual dwellings ranges widely. By applying four different analytic approaches, we identify several determinants of high energy savings, including the following: weatherizing high energy users; installing attic, wall, and floor insulation; insulating water heaters, installing low-flow showerheads, and reducing hot water temperatures; curing distribution system problems; and replacing inefficient heating systems. The installation of storm windows and doors, on the other hand, generally is associated with inferior savings. This paper demonstrates the value of using alternative but complementary qualitative and quantitative methods to identify determinants of Program effectiveness.