This report analyses approaches to managing environmental compliance monitoring and enforcement in several OECD countries with decentralised systems of environmental governance. It focuses principally on strategies and instruments for promoting consistency in the implementation of national environmental law. The report reviews in detail the experience of Spain, Sweden, Switzerland and the United States and draws on examples from several other countries. Three programmatic elements of environmental enforcement are key to ensuring its consistency: the targeting of compliance monitoring; the selection of an enforcement instrument and the timeliness of noncompliance response; and the size of monetary penalties for non-compliance. Accurate and complete information on the performance of sub-national and local competent authorities is an important prerequisite for the evaluation of nationwide consistency of enforcement. To address these issues, OECD countries employ a range of mechanisms of institutional interaction: “vertical” (between different administrative levels) as well as “horizontal” (between competent authorities at the same level). The report presents multiple examples of the application of each mechanism in different decentralised systems. It analyses these good practices and suggests several ways to use them to ensure consistency in the implementation of the main elements of enforcement programmes.