The role of farmers in technology generation and diffusion has long been identified as a key dimension in publicly funded international agricultural research. This thesis uses a case study approach to document actual research and diffusion practices andlheir outcomes, and from these it draws conclusions for research policy. The thesis compares the effects of new technology, the research processes that generated it, and the diffusion processes that promoted and disseminated it, in three countries. Particular attention is given to farmer participation and related issues deemed critical to effective research and diffusion. The technology in question is True Potato Seed, a radical alternative means for potato propagation to tuber seed, researched and promoted by the International Potato Centre in collaboration with national research institutes since 1978. The case study countries are India, Egypt, and Indonesia. Extensive quantitative and qualitative farmer surveys provide the first detailed assessment of TPS benefits, their distribntion, and likely TPS adoption. Secondary data, documentation, and in-depth interviews with key actors permit an analysis of the significant activities, decisions, and players that shaped TPS research and diffusion processes. The farmer survey evidence indicates that TPS outcomes are problematic in each country. It also reveals cases of inadequately justified TPS research and promotion largely due to a poor awareness by scientists and research managers of farmers' conditions. This thesis demonstrates that a powerful countervailing force to the pursuit of unproductive research lies in the early and effective involvement of farmers in the research process. This allows for critical weaknesses to come to light after experimentation by farmers under their own conditions of production. The inevitable geographical and institutional decentralisation that this entails, fosters a greater level of research responsiveness and an environment in which alternatives to formal extension, including farmer dissemination networks, may be stimulated and encouraged where appropriate. However, regardless of the rhetoric regarding farmer participatory approaches at an organisational level, the extent to which these are borne out in practice depends largely upon the philosophies of the key individuals who shape and direct research and promotion processes.