This paper presents case-study results and aggregate data to evaluate the impact of research in African agriculture. Of 32 case studies, all but eight report annual returns over 20% and many are far higher, with most gains arising in the late 1980s and 1990s. Spurred by policy reforms and changing incentives, these innovations have led to sustained growth in aggregate cereal crop yields since 1985. Africa's belated 'green revolution' is based on new varieties (often with early maturation for drought escape), complemented by new management techniques (typically labor-intensive efforts to conserve soil moisture and build soil fertility). © 1998 Elsevier Science B.V. All rights reserved.