Why are agricultural yields so low and growing so slowly in Africa? Using simple models, this paper proposes a research program to uncover the reasons for the low levels of input intensity and slow pace of technological innovation in African agriculture. Attention is focused on the relative prices of output and factors of production, credit constraints, imperfect insurance, learning externalities and insecure property rights. A set of research projects is proposed to examine these hypothesized sources of low and slow growing productivity.