The use of mixed systems for agricultural and forestry production is by no means new. In pure agricultural systems the sowing of mixed annual crops has been a common farmer response to risk from climatic uncertainties to the need for diversity of food crops and to pragmatic observations of beneficial interactions between species. In the humid tropics, whether highland or lowland, sophisticated tree-crop mixtures have been developed over centuries using many species and where a wide variety of benefits are obtained. In the drier parts of Africa trees have formed important components of grazing systems, or have been left deliberately for their benefits in cultivated land. A simpler system, the use of agriculture crops for assisting in the establishment of forest plantations (taungya) is widespread, and in Africa as elsewhere shifting agriculture provided, in the past, a sustainable land-use system which is only now breaking down under the pressure of increasing populations.