This paper discusses the role that impact evaluations should play in scaling up. Credible impact evaluations are needed to ensure that the most effective programs are scaled up at the national or international levels. Scaling up is possible only if a case can be made that programs that have been successful on a small scale would work in other contexts. Therefore the very objective of scaling up implies that learning from experience is possible. Because programs that have been shown to be successful can be replicated in other countries while unsuccessful programs can be abandoned, impact evaluations are international public goods, thus the international agencies should have a key role in promoting and financing them. In doing this, they would achieve three important objectives: improve the rates of return on the programs they support, improve the rates of return on the programs other policymakers support by providing evidence on the basis of which programs can be selected, and build long-term support for international aid and development by making it possible to credibly signal what programs work and what programs do not work. The paper argues that considerable scope exists for expanding the use of randomized evaluations. For a broad class of development programs, randomized evaluation can be used to overcome the problems often encountered when using current evaluation practices.