According to common-sense morality, the difference between doing and allowing harm matters morally. Doing harm can be wrong when merely allowing harm would not be, even if all other factors are equal: the level of harm is the same, the agent’s motivation is the same, the cost to the agent of avoiding countenancing harm is the same, and so on. Suppose that you have accidentally swallowed poison and you need to get to hospital urgently. It might be permissible for you to refuse to stop and help if you spot Sarah drowning, but impermissible for you to push her into a river to clear your way to the hospital. Without this moral distinction between doing and allowing, it seems likely that our everyday morality would look very different. Treating doing and allowing harm as equivalent seems to leave us with a morality that is either much more permissive than we normally think it is (permitting us to do harm to others to avoid personal sacrifices) or much more demanding (requiring us to prevent harm to others even at great personal sacrifice). Yet the moral significance of the distinction is highly controversial. When serious harm to others is at stake, it may seem puzzling that it should matter whether the harm is done or merely allowed. Powerful critics have argued that the distinction is morally irrelevant. Others have charged that the distinction itself falls apart under scrutiny: our intuitions about whether behavior counts as doing or allowing harm do not reflect any clear, nonmoral distinction. Much of the early contemporary debate on the moral relevance of the distinction between doing and allowing harm focused on appeals to intuitions. We are asked to examine “contrast cases” in which all others factors are supposedly held constant. However, appeals to intuitions are of limited use. They may establish whether we treat the distinction as morally relevant, but they cannot show whether we ought to do so. The real challenge for a defender of the doing/allowing distinction is to provide a clear analysis of the distinction and a convincing argument that, under this analysis, the distinction connects appropriately to more fundamental moral concepts. This article maps out the philosophical literature on the analysis and moral significance of the distinction between doing and allowing harm, from the beginnings of contemporary interest in the issue in the 1960s and 1970s to recent trends and developments.