The rapid rise of the responsibility to protect provides us with a unique opportunity to consider the impact of a decade or so of determined norm entrepreneurship. The responsibility to protect has not yet become a binding norm of international law, and in this article we examine what factors are holding back or promoting this development. We draw on an 'interactional' account of international law, which focuses on three inter-locking elements. First, legal norms are social norms and as such they are connected to social practice – they must be grounded in shared understandings. Second, what distinguishes law from other types of social ordering is not so much form or pedigree, as adherence to specific criteria of legality. When norm creation meets these criteria and, third, is matched with norm application that also satisfies the legality requirements, international law will have legitimacy and generate a sense of commitment among those to whom it is addressed. After highlighting key steps in the norm building process so far, from the 2001 report of the International Commission on Intervention and State Sovereignty to the General Assembly debate in 2009, we offer a brief sketch of our theoretical framework, and employ it to examine the trajectory of the responsibility to protect norm, concluding with an assessment of its current and potential status as binding law. Although the responsibility to protect, including its potential for the collectively authorized use of force, is increasingly supported by globally shared understandings, the norm falls short on several of the legality criteria. Furthermore, given the inconsistent practice on protective use of force, no practice of legality can be said to have evolved. Proponents of the norm face a lot of hard work ahead.