Over the past 50 years public agricultural research has contributed enormously to humanity, enabling the supply of food to grow faster than demand in spite of a rapidly growing population, income growth, and shrinking natural resources. Nonetheless, in many countries we see waning public support for agricultural R&D, especially in Africa, a diversion of research resources from farm productivity towards other agendas, and early warning signs of a slowdown in agricultural productivity. The world has continued to collectively underinvest in agricultural R&D because of domestic and international market failures associated with appropriability problems. Governments have failed to effectively address these problems, often doing too little, too late. This tragedy of the international commons may be getting worse. In the past, developing countries benefited considerably from technological spillovers from developed countries, but because of changes occurring in developed countries, spillovers from developed countries may not be available to developing countries in the same ways or to the same extent. In this article, the factors contributing to persistent global underinvestment in agricultural R&D are described from a developing-country perspective, estimates of agricultural R&D spending trends are presented, and incentive mechanisms for increasing rates of investment in agricultural R&D are described and assessed.