During the past two decades or so, rural population in Africa has increased slowly while urban population has grown dramatically. The hugely increased urban demand for cereals and pulses (which produce crop residues for livestock) and for livestock products is now the main force stimulating mixed farming systems in the semi-arid and sub-humid areas of sub-Saharan Africa. Grazing land has diminished, crop residues are becoming a more important element in raising livestock and fattening penned livestock has become profitable. The changes in land use, land tenure and the shift of livestock raising southwards in West Africa are illustrated. Farmers' adaptation to rapidly changing markets for their products and the factors of production are illustrated with examples from Senegal, Nigeria, Niger, and, by way of contrast, Kenya. The main challenges this sets to agricultural scientists are described. The livestock element in mixed farming system now requires careful economic analysis and participative research if scientists are to meet the evolving needs of farming as the urban sector enlarges.