Résumé: The reduction of environmental and health risks from pesticide use is a major challenge for agriculture and on top of the agricultural policy agenda. Despite substantial efforts, current pesticide policies have not reached their reduction targets. Recent research shows that targeting farmers’ application behavior could be key for advancing pesticide policies. However, our theoretical understanding of the farmers’ decision-making process is limited and empirical studies on the main drivers of inefficient decisions are missing. The interdisciplinary PESTEFFECT project will i) advance our understanding of farmers’ pesticide use decisions, ii) quantify impacts of the main drivers of inefficient decisions, and iii) identify risk reduction potentials on a regional level. In the project, we will i) develop a conceptual and empirical model of farmers’ pesticide use decisions that integrates concepts from agricultural economics, agronomy, and ecology, ii) provide the first empirical analysis of economic and environmental effects of inefficient pest management decisions using an econometric model and iii) provide temporally- and spatially explicit assessments of risk reduction potentials on a regional level using multi-layered maps. For our analysis, we will use a unique, long-term dataset on farmers’ management decisions, environmental conditions, and biodiversity (losses) from Western France. Results will support the development of pesticide policies targeting inefficient pest management decisions and the creation of guidelines for farmers and farm advisors. The project will thus contribute to a more sustainable agriculture in Europe.