Hardin's parable of the commons was influential out of all proportion to its evidential content, which was none: it described a hypothetical pasture and drew a conclusion about human institutions. Ostrom's response was to look. What she and her collaborators found across irrigation systems, fisheries and forests was that communities managing a shared resource frequently devise rules that prevent depletion, and that the successful cases share identifiable features — boundaries that are clear, rules made by the people bound by them, monitoring by those with a stake, sanctions that escalate. The finding did not show that commons never collapse; many do. It showed that collapse is a possible outcome rather than a theorem, and that the conditions distinguishing the outcomes are empirical and to some extent designable. Empirical work replaced a parable, which is a modest achievement with unusually wide consequences.