The authors analyze the considerations that lead policymakers to undertake extension investments as a key public responsibility, as well as the complex set of factors and intra-agency incentives that explain why different extension systems' performance vary. The authors provide a conceptual framework outlining farmers' demand for information, the welfare economic characterizations of extension services, and the organizational and political attributes that govern the performance of extension systems. They use the conceptual framework to examine several extension modalities and to analyze their likely and actual effectiveness. Specifically, the modalities reviewed include "training and visit" extension, decentralized systems, "fee-for-service" and privatized extension, and farmer-field-schools. The authors also discuss methodological issues pertaining to the assessment of extension outcomes and review the empirical literature on extension impact. They emphasize the efficiency gains that can come from locally decentralized delivery systems with incentive structures based largely on private provision that in most poorer countries is still publicly-funded. In wealthier countries, and for particular higher income farmer groups, extension systems will likely evolve into fee-for-service organizations
