This article investigates whether international organizations can be held responsible under international law when they fail to act. It aims to conceptualize the notion of ‘omission’ in the international law on the responsibility of international organizations and does so in four broad steps. First, a discussion of the most well-known failure (the United Nations’ refusal to intervene in the Rwandan genocide in 1994) suggests that there is a need to conceptualize the omission and to reflect on the sort of factors that may cause a failure to act. Second, the article investigates how omissions have been addressed in the literature and in the codification of the law on responsibility and finds that little attention has been paid to omissions, and where attention has been paid, it has been limited to viewing the omission as the mirror image of the act. Third, the article addresses as one element of a relevant concept of omission that the organization must be in a position to act, and, fourth, it establishes the basis of an obligation to act in some circumstances on the basis of the organization’s mandate, thus introducing a version of what can be called ‘role responsibility’ into international law.