Integrated Pest Management (IPM) programs based on the Farmer Field School approach are being implemented in many countries. Their benefits have been recognized by a broad range of stakeholders, including farming communities, local and national governments, NGOs and donors, who are now supporting such programs. Substantial investment in this approach calls for comprehensive documentation of its impact and sustainability. This study reviews twenty-five impact studies and discusses the overall results. Most of the ongoing programs have conducted impact evaluations of a kind. These varied in focus, approach, methodology and robustness. Results, however, commonly remained buried in project reports. Impact evaluation of the IPM Farmer Field School has proven to be complex because of methodological obstacles, because of the range of immediate and developmental impacts, and because of different perspectives of stakeholders. Consequently, there is no agreed conceptual framework for measuring impact.