This paper analyses the effect of National Numerical Fiscal Rules (FRs) upon fiscal discipline in 74 developing countries over the period 1990-2007. It is the first study that assesses the impact of FRs on budgetary outcomes while controlling for the self-selection problem. It finds that the effect of FRs on structural fiscal balance is significantly positive, robust to a variety of alternative specifications, and varies with the type of FRs. It also finds that the treatment effect differs according to countries characteristics: number of FRs, time length since FRs adoption, presence of supranational FRs, government fractionalisation and government stability.