There are two basic forms of legal and regulatory cooperation in world politics: interstate and transgovernmental. The former involves states behaving as unitary actors, facing the world as integrated units and each speaking with one voice in its interactions with other unitary states. The latter occurs when cross-border cooperation takes place directly between governmental subunits of different states, such as courts and regulatory agencies. But why is legal and regulatory cooperation among some states and in some issue areas principally interstate, while among other states and in other issue areas it is primarily transgovernmental? The principal goal of this article is to contribute to the scholarship on transgovernmental legal and regulatory networks and transgovernmentalism in general by proposing some possible answers to this question. To accomplish this, the article uses two analytical tools from the discipline of political science: the concept of transgovernmental relations and the theory of rational institutional design. It applies these tools to develop a rational design theory of transgovernmentalism aimed at explaining the conditions under which legal and regulatory cooperation is more likely to be transgovernmental versus interstate. The article probes the theory's plausibility using a case study of E.U.-U.S. merger review cooperation.