This Article argues that networks of private contracts serve a public regulatory function in the global environmental arena. These networks fill the regulatory gaps created when global trade increases the exploitation of global commons resources and shifts production to exporting countries with lax environmental standards. As critics of trade liberalization have noted, public responses often are inadequate to address the attendant environmental harms. This Article uses empirical data to examine how private contracting regulates firm behavior, focusing on supply-chain contracting. The Article shows that more than half of the largest firms in eight retail and industrial sectors impose environmental requirements on their domestic and foreign suppliers. This contracting, which the Article terms "the new Wal-Mart effect," reduces externalities by translating a complex mix of social, economic, and legal incentives for environmental protection into private contractual requirements. After demonstrating that private environmental contracting is an important part of global environmental governance, the Article examines the efficacy and accountability of this regime. The Article concludes that the private contracting regime often is preferable to the alternatives: lax national and international regulation of firms in many exporting countries, and markets that lack private environmental contracting. Finding much promise in the private contracting regime, the Article concludes by suggesting new strategies for governments, nongovernmental organizations, and firms.