In recent years, the development and the use of engineered nanomaterials have generated many debates on whether these materials should be part of the new or existing regulatory frameworks. The uncertainty, lack of scientific knowledge and rapid expansion of products containing nanomaterials have added even more to the regulatory dilemma with policy makers and public/private actors contenting periods of both under and over regulation. Responding to these regulatory challenges, as well as to the global reach of nanotechnology research and industrial needs, governance arrangements beyond the state have addressed the challenge head-on. This article focuses on the governance arrangements of the International Organization for Standardization (ISO), which has led to the development of numerous “horizontal anticipatory standards” with an important role in setting the foundation for science, technology and market development. During the course of its operation ISO has broadened its scope to address not only technical issues related to the concept and the size of nanomaterials but also broader aspects of the technology, including health, environment and safety issues. The increasing relevance of the ISO to regulate economic relations and achieve certain public policy goals has given rise to many concerns about its legitimacy. The important questions are whether these governance arrangements may be deemed as being legitimate and where this legitimacy is derived from? What are the main sources of legitimacy at the transnational level and how we can apply them to analyse nanotechnology standardization? This article provides concise answers to these questions. It focuses at the normative concepts of democratic and scientific legitimacy and explores the institutional structures and processes by which nanotechnology standards are established.