Virtually everyone believes that we have a duty to rescue fellow human-beings from serious danger when we can do so at small cost to ourselves – and this often forms the starting point for arguments in moral and political philosophy on topics such as global poverty, state legitimacy, refugees, and the donation of body parts. But how are we to explain this duty, and within what limits does it apply? It cannot be subsumed under a wider consequentialist requirement to prevent harm. Nor can it be understood as a duty of social justice that citizens owe to one another under a social contract for mutual protection. Instead it is a sui generis duty of justice that arises from the direct physical encounter between rescuer and victim, and is accordingly limited in scope. It is unconditional, in the sense that it cannot be voided either by reckless behaviour on the part of the rescuee, or by her unwillingness to reciprocate if called upon to do so. However the simplicity of the duty evaporates when multiple potential rescuers are present. Here responsibility lies with the collective as a whole until it is assigned by a fair procedure to individual members. Each individual is required as a matter of justice to discharge that share, but not more, though in the case that others do not comply, he will have a reason, and sometimes a humanitarian duty, to take up the slack.