This article studies the preventive pillar of the responsibility to protect (R2P) norm and focuses on its normative development, robustness, normative fit and its chances of becoming a legal norm, based on the sources of international law and the theory of interactional law. This article argues that prevention is the pillar of R2P with the widest level of international support and is based on increasingly deeper and more precise shared understandings. The institutionalisation process and a norm cascade - possibly leading to internalisation - are materialising simultaneously while the interpretation and application of the norm are still being contested. To be more robust and more effective in practice, the norm ‘responsibility to prevent’ has to gain in specificity. Prevention of mass atrocities finds increasing support in the sources of international law. For an interactional legal norm to emerge, the exact content and division of responsibilities has to become clearer, officials have to act in line with the norm and practices of legality have to emerge.