Over the past years, two important challenges to traditional conceptions of international law have come to the fore: pluralism and informality. Pluralism refers to the fact that global society is characterized by multiple legal systems that frequently overlap. Informality refers to the increasing relevance of non-traditional actors, processes and outputs in the realm of international law. Two recent books provide important contributions to enhancing our understanding of these challenges. In the first book, Nico Krisch presents a powerful plea that in a ‘postnational’ context legal pluralism is to be preferred over its alternative, transferring constitutionalism to the global level. In the second book, Joost Pauwelyn, Ramses Wessel and Jan Wouters offer a collection of essays that seek to further knowledge of the concept, legal nature, impacts and accountability of ‘informal international lawmaking’. In reviewing these two books, this review essay seeks to identify avenues for further inquiry in the area of transnational environmental law.