Appropriate sites for new irrigation development are increasingly hard to find, and many countries are choosing to rehabilitate existing systems instead. This study assesses the effectiveness and replicability of the Northeast Small Scale Irrigation Project (NESSI), which involved rehabilitation of seven medium-scale irrigation systems in Northeast Thailand. Despite its excessive complexity, the project's management structure has largely been effective at the national, provincial, and project levels. NESSI has also made a generally positive contribution to farmer participation in water management by forming chaek groups and reviving water user associations. However, farmer involvement has been limited to the on-farm system, primarily the farm ditch. Extension activities have been less successful, due mainly to a reliance on traditional techniques and a failure to respond to the changing economic climate in the region. Also, marketing assistance was provided only prior to completion of the irrigation sites, and by the time farmers were able to actually produce additional dry season crops, was no longer available. Technically, NESSI's on-farm engineering design was effective, but its exceptional cost limits the areas that can be rehabilitated using this design. Economically, NESSI has been a success, yielding a return rate greater than 10% on four sites, and helping increase family incomes 20-50% in three areas. A major factor in this success has been the Thai government's encouragement of private investment in the region. Includes recommendations for the next phase of medium-scale irrigation systems development in Northeast Thailand.