Agricultural research and extension programs have been built in most of the world's economies. A substantial number of economic impact studies evaluating the contributions of research and extension programs to increased farm productivity and farm incomes and to consumer welfare have been undertaken in recent years. This chapter reviews these studies using estimated rates of return on investment to index economic impacts. In almost all categories of studies, median (social) estimated rates of return are high, (often exceeding 40 percent) but the range of estimates was also high. The chapter concludes that most of the estimates were consistent with actual economic growth experiences.