Field observations have led many people to believe that beneficiary participation in decision making can contribute greatly to the success of development projects. When people influence or control the decisions that affect them, they have a greater stake in the outcome and will work harder to ensure success. But the evidence supporting this reasoning is qualitative so that many practictioners remain skeptical. Three questions need to be addressed: to what degree does participation contribute to project effectiveness? which beneficiary and agency characteristics foster the process? and, if participation does benefit project outcomes, how can it be encouraged through policy and project design? To answer these questions, researchers studied evaluations of 121 completed rural water supply projects in forty-nine developing countries around the world. The results show that beneficiary participation contributes significantly to project effectiveness, even after statistically controlling for the effects of 17 other factors. The basic conclusion of this study is that rural water projects must be fundamentally redesigned in order to reach the one billion rural poor who lack a sustainable water supply. Redesign must encompass a shift from supply-driven planning to demand-responsive, participatory approaches to ensure beneficiary participation, control, and ownership.