Over the past two centuries, public environmental regulation (PER) has been progressively supplemented by private transnational regulation (PTR), creating a hybrid environmental governance regime. A five-category typology is developed to describe the ways in which international and national PER interact with private forms of environmental regulation. We then analyze the policy considerations that are relevant to the design of such hybrid regimes and various forms of interaction. Next, we describe two case studies that demonstrate the diversity of interactions between PER and PTR in a single regime. The case of sustainability reporting illustrates how public law builds on the expertise developed by private organizations as gradually more reporting obligations are incorporated into public law. The case of sustainable forest management regulation is somewhat more mixed, reflecting a tendency for increased state intervention, which led to partial suppression of PTR.