This article analyzes the development of environmental regulation in the European Union from the perspective of comparative federalism. It presents a theory of regulatory federalism that explains how the basic institutional structures of federal-type polities shape the development of regulatory policy. The article assesses the theory by systematically comparing the development of environmental regulation in the EU, the US, Canada and Australia. The analysis suggests that the EU's institutional structure encourages the development of a US-style pattern of regulation, characterized by detailed, non-discretionary rules and a litigious approach to enforcement.