This report contains a review of the literature on the role of agricultural research and development in fostering innovation and productivity in agriculture. The review seeks to clarify concepts and terminology used in the area, provide a critical assessment of approaches found in the literature, report main results, and draw inferences. A key finding is that the social rate of return to investments in agricultural R&D has been generally high. Specific findings differ depending on methods and modelling assumptions, particularly assumptions concerning the research lag distribution, the nature of the research-induced technological change, and the nature of the markets for the affected commodities.