This chapter sets out to show how, on the plains of Sumatra and Kalimantan, fallow management has provided an intermediate stage in evolution from shifting cultivation of upland rice to a more sustainable complex agroforestry system based on rubber. This evolution will be described from two perspectives: farmer-generated innovations, using indigenous knowledge, and their adoption of innovations from outside. These two processes are fundamentally different. They arise from farmers' responses to both market opportunities and the need to adopt more productive and competitive land-use systems. Introduction of rubber to the study area, and the subsequent development of local knowledge about its integration into farming systems, has led to the evolution of sustainable agroforestry practices. A long and important transitionary stage in this evolution has involved gradual improvements to the traditional management of the long, 30-year secondary forest fallow.