Enforcement of environmental laws is one of the key issues in the debate over the relationship between trade and environment in the North American Free Trade Agreement (NAFTA). Without uniformly strong enforcement in all three NAFTA nations, there is the potential for increased migration of “dirty” industries to nations with lax enforcement, and for increased environmental degradation. Furthermore, industries subject to lax enforcement do not have to internalize environmental compliance costs and so have a competitive advantage over their international rivals. This article discusses various approaches to encouraging enhanced enforcement of environmental laws as one component of a supplemental agreement to the NAFTA. The article focuses on the enforcement approach advocated by the Center for International Environmental Law and the Defenders of Wildlife. First, this article sets forth the enforcement proposals put forward by the governments of the three Parties, by the U.S. Congress, and by the environmental community. The article then analyzes the two basic approaches to enforcement found in these proposals, concluding with suggestions for the most effective ways to incorporate environmental enforcement provisions in NAFTA side agreements.