Failures of compliance with European Union (EU) directives have revealed the EU as a political system capable of enacting laws in a wide range of different policy areas, but facing difficulties to ensure their actual implementation. Although the EU relies on national enforcement agencies to ensure compliance with the EU legislation, there is scarce analysis of the differential deterrent effect of national enforcement in EU law compliance. This article examines the enforcement of an EU water directive, the Urban Waste Water Treatment Directive, in Spain and the UK. It focuses on the existing national sanctions for disciplining actors in charge of complying with EU requirements, and on the actual use of punitive sanctions. The analysis shows that a more comprehensive and active disciplinary regime at the national level contributes to explain a higher degree of compliance with EU law. The article calls for a detailed examination of the national administrative and criminal sanction system for a more comprehensive understanding of the incentives and disincentives to comply with EU law at the national state level.