In the second half of the twentieth century, many developing countries adopted broad social assistance programs, like food subsidies, ostensibly designed to help poor people. Their effectiveness was mixed and, unfortunately, many of these expensive programs did not make much difference in the lives of poor people, much less help them climb permanently out of poverty. In the 1990s Mexico took a completely new approach. It launched a social program— PROGRESA—that was revolutionary in two ways. First, PROGRESA aimed to integrate interventions in health, education, and nutrition simultaneously, based on an understanding that these dimensions of human welfare are interdependent and that poor health, education, and nutrition are both causes and consequences of poverty. Second, PROGRESA was designed from the beginning to be continually evaluated and improved, so that it would become ever more effective at improving the well-being of Mexico’s poorest people. From 1998 to 2000 the International Food Policy Research Institute (IFPRI) assisted the PROGRESA administration in evaluating the program. This research resulted in a series of IFPRI reports, synthesized here, on aspects of PROGRESA’s performance. The evaluation not only highlighted areas of success, but also suggested needed improvements in the program. On the one hand, for example, the research showed that PROGRESA has helped keep poor children in school longer, improved the health of young children and adults, increased women’s use of prenatal care, and improved child nutrition. On the other hand, the evaluation revealed that PROGRESA could have a greater impact on school enrollment by focusing on attendance in secondary schools—the stage at which many poor children drop out. In the election of 2000, the people of Mexico voted a new party into power. Yet, faced with evidence of PROGRESA’s effectiveness, the new government decided to keep the program (renamed Oportunidades) and to make needed improvements in its operation. IFPRI’s research on PROGRESA has advanced our knowledge about policy steps that governments can take to improve the capacities of poor people, who may require interventions in several areas to make real headway in overcoming poverty. In the meantime comparable programs have been tested or implemented in other countries, including Brazil, Honduras, and Nicaragua, and it would be promising to explore their adaptation in African and Asian countries as well. It is our hope that research of this kind will encourage replications, adaptations to country circumstances, and rigorous evaluation of social programs in many developing countries.