This Chapter addresses the question why international actors would opt for more informal settings and output and what the consequences of this choice are in terms of elements that are traditionally perceived as being weak in traditional law, such as legitimacy and compliance. This question relates to the choice for a certain governance arrangement. As this is not a legal question per se, this Chapter uses insights offered by other disciplines to shed some more light on this choice. It will make an attempt to contribute to the debate by pointing to the choice of States (or other actors in the public sphere) to move from formal to informal international decision-making as well as to some consequences of this choice. What will be termed a ‘turn to informality’ will be first of all be approached theoretically by assessing reasons for actors to choose certain governance arrangements. Using a social science approach, these theoretical assumptions will then be assessed in the framework of a case study on the International Organization for Standardization (ISO) when dealing with the regulatory governance of nanotechnologies. This Chapter could also be seen as a plea for new approaches linking theoretical analysis and empirical studies in international institutional law. The turn to informality in the real world can only lead to an acceptance in international legal scholarship when empirical findings and insights from social sciences are taken on board.