This article identifies ‘the tragedy of the commons’ as an essentially aggregative harm and considers what agents in such a scenario owe to one another. It proposes that the duty to take reasonable precautions requires that agents make efforts to establish collective solutions to any essentially aggregative harm to which they would otherwise contribute. Baylor Johnson has argued that the general obligation to promote the common good requires that agents make efforts to establish a collective agreement to avert a potential tragedy of the commons. This article agrees with Johnson's analysis but argues that there are also negative duties, which give commons users additional moral reason to take collective action to prevent the destruction of the utility of the commons. These duties link commons users to resolving this particular problem and provide grounds to demand that they make greater efforts to prevent the destruction of the commons compared to bystanders. The article suggests that fulfilling negative duties can require establishing collective institutions with the power to regulate human action and interaction. This analysis could be applied to a range of social problems which result from the aggregation of actions and practices.