This study asks: which specific policy efforts contributed most to the reduction in Amazon deforestation? The paper, which evaluates the impact of monitoring on deforestation, focuses on DETER, a satellite-based system for real-time detection of deforestation. DETER is the key tool for targeting law enforcement activities in the Brazilian Amazon. The study shows that DETER was the main driver of the 2000s deforestation slowdown: deforestation observed from 2007 through 2011 was 75 per cent smaller than it would have been in the absence of monitoring and resulting fines. However, more stringent monitoring had no impact on municipality-level agricultural production.