PIP: In the last half century, the technological development of agriculture has dramatically changed farming. On the positive side, a more stable and abundant food supply has resulted; on the negative side are more environmental degradation, more dependence on fossil energy, and a lower energy efficiency. Farmers operating in developed countries abandoned traditional techniques of production to keep their income competitive with that of other sectors of society. The result was large monocultures that rely heavily on technical inputs. Subsistence farming systems are economically not sustainable when these societies interact with more developed socioeconomic systems. However, the high-technology agricultural techniques depend on nonrenewable stocks of oil and have negative environmental impacts: soil erosion, reduced biodiversity, chemical contamination of the environment by fertilizers and pesticides, and mining of groundwater. Hence, the current intensive agriculture based on heavy technological subsidies of fossil energy is ecologically not sustainable. The 2 major sources of energy used by human society are solar energy and fossil or nonrenewable energy, which represents more than 90% of the exosomatic energy used in the United States and other developed countries. Fossil energy is used to overcome the ecological constraints limiting food output. The rapid growth in the world population is associated with the maximum expansion of fossil energy use. Regarding strategies of energy use in world agriculture, in the US fossil energy is mainly used to boost farmers' productivity, while productivity per hectare is not as much a concern as in Europe. Because of the demographic explosion experienced in the last decades, Africa will experience declining food supplies, increasing poverty, and increasing environmental degradation. The level of renewable energy consumption that will be enjoyed by a future sustainable society will lie below the one reached today by developed countries and above the one typical of preindustrial societies, which relied completely on photosynthesis.