Federal social programs are rarely evaluated to determine whether they are actually accomplishing their intended purposes. As part of its obligation to spend taxpayers’ dollars wisely, Congress should mandate that experimental evaluations of every federal social program be conducted. The evaluations should be large-scale, multisite studies to guard against mistakenly assuming that a program that works in one location or with one population will automatically work in other situations. Congress should place substantially less emphasis on funding evaluations based on less rigorous types of research designs, because their conclusions are much less reliable. Finally, Congress should exercise strict oversight to ensure that the evaluations are conducted and the results reported in a timely manner.