This paper for the launching of the journal Transnational Environmental Law first sets forth a concept of transnational environmental law that encompasses but is broader than international environmental law. When we speak of transnational law and legal process, we are concerned with the migration and impact of legal norms, rules and models across borders. Such migration can occur through the mediation of international law and institutions, or through the impact of unilateral legal developments in one jurisdiction that affect behavior in others. The paper then discusses the importance of assessing transnational environmental law in light of the constraints facing consent-based international environmental law, examines the tradeoffs between transnational and international environmental law from the perspective of legitimacy, and concludes by discussing the important but delicate relation of international law to transnational environmental law as both a check and a consolidator. International law should guard against the self-serving unilateral use of transnational environmental law, but it should do so in a way that preserves (and does not shut off) the dynamic, responsive character of the transnational environmental law process. Otherwise international law itself will be delegitimized.