A useful evaluation of an education program is one that adds to the body of timely, relevant evidence to increase the likelihood that policy decisions improve program performance. This definition focuses on performance rather than change and recognizes that studies can't guarantee the use of their findings but can only increase the likelihood of such use. Adding to the body of sound program evidence is the evaluators' main method for influencing programs. Unprecedented congressional demand for scientifically sound evaluation evidence will lead investigators to conduct experiments and systematic reviews to rigorously answer causal program questions. But not all questions related to improving education program performance are causal, and other useful methodologies utilize scientifically valid, large-scale surveys and case studies to evaluate program outcomes and better understand implementation. The experience of the U.S. Department of Education demonstrates that well-designed, timely evaluations using scientifically valid methodologies tailored to important questions can make important contributions to legislative, budget, and administrative decisions to improve the performance of government-funded programs. [This article was produced by SAGE Publications, Thousand Oaks, CA.]