In the twenty-first century, it is evident that world agricultural systems will have to supply sufficient food for a population somewhere between 7.5 and 12 billion. Projections for world agriculture in the first half of the twenty-first century very widely, largely depending on assumptions about yield growth. An investigation of the patterns of yield growth for major cereal crops offers evidence that the pattern is logistic, implying that an upper limit to yields is being approached. This pattern is consistent with ecological limits on soil fertility, water availability, and nutrient uptake. It is also evident that current agricultural production is imposing serious strains on ecosystems, with widespread soil degradation, water overdraft and pollution, and ecological impacts such as loss of biodiversity and the proliferation of resistant pest species. The issue therefore is not simply the balance of supply and demand in agriculture. It is the need to develop ecologically sustainable agricultural systems which can provide an agricultural output about twice present aggregate levels (allowing for per capita growth in consumption). This level of output would support a population of about 8 billion. In addition, a population policy which can avert any much higher growth is essential. Evidence exists to show that ecologically sustainable cropping systems can supply overall outputs comparable to intensive high-input agriculture. (The measure of overall output is distinct from the more commonly used measure of single-crop yields.) This evidence, however, is more compelling for temperate zones with good soils. Much more research is needed on sustainable agriculture for tropical and arid zones. Agricultural policies need to be reformulated to meet the new goal of sustainability. These sustainable agriculture policies must be developed in tandem with population policies to ensure that population growth remains in the lower ranges of current projections.