Informal international lawmaking is a term used to describe the process by which new international legal norms are created outside of the normal fora for negotiation. The outcomes of these processes are not codified in treaties or conventions and thus remain non-binding. They are alsocharacterized by the involvement of non-traditional actors, such as regulators and agencies. This book critically assesses the concept of informal international lawmaking, its legal nature, and impact at the national and international level. It examines whether it is on the rise, as is oftenclaimed, and if so, what the implications of this are. It addresses what actors are involved in its creation, the processes utilized, and the informal output produced. The book frames informal international lawmaking around three axes: output informality (novel types of norms), process informality (norm-making in networks outside international organizations), and actor informality (the involvement of public agencies and regulators, private actors, andinternational organizations). Fundamentally, the book is concerned with whether this informality causes problems in terms of keeping transnational lawmaking accountable. By empirically analysing domestic processes of norm elaboration and implementation, the book addresses the key question of how tobenefit from the effectiveness of informal international lawmaking without jeopardizing the accountability necessary in the process of making law.