Cassava (Manihot esculenta Crantz) is an important food crop in many tropical countries.  The crop is known to grow well in acid and infertile soils, but can cause serious erosion when grown on slopes.  To help farmers increase their yields, a farmer participatory project was conducted in Thailand, Vietnam and China from 1994 to 2004. Cassava farmers in selected villages were asked to discuss their cassava production practices and diagnose their main constraints using various participatory diagnosis techniques.  Once the main problems were identified and prioritized, farmers were encouraged to conduct simple trials on their own fields to develop the most appropriate solutions.  Farmers selected the type of trials, as well as the treatments to be tested.  Project staff then helped farmers to set out the trials and establish the treatments.  Farmers managed the trials but project staff visited regularly to discuss possible problems.  At time of harvest, field days were organized so other farmers from the village and from surrounding areas could visit the trials, evaluate the treatments and discuss the results.  The best treatments were often retested for 1-2 more years to confirm the results, after which many farmers started adopting the most suitable practices.  During a five year period, from 1999 to 2004, over 1,100 trials were conducted in a total of 99 villages, resulting in widespread adoption of new high-yielding varieties and improved agronomic practices.  This in turn resulted in marked cassava yield increases in Asia, from 12.9 t/ha in 1994 to 19.85 t/ha in 2008.  Involving smallholder farmers directly in the diagnosis of problems and in the testing of potential solutions, was found to be a very effective way to achieve widespread adoption of new technologies.