Agricultural research has transformed agriculture and in doing so contributed to the transformation of economies. Economic issues arise because agricultural research is subject to various market fail ures, because the resulting innovations and technological changes have important economic consequences for net income and its distri bution, and because the consequences are difficult to discern and attribute. Economists have developed models and measures of the economic consequences of agricultural R&D and related policies in contributions that relate to a very broad literature ranging across production economics, development economics, industrial organiza tion, economic history, welfare economics, political economy, econo metrics, and so on. A key general 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 modeling assump tions, particularly assumptions concerning the research lag distribu tion, the nature of the research-induced technological change, and the nature of the markets for the affected commodities.