Abstract: The concept of ‘irreversibility’ plays a large role in many domains, including public health, medical practice and environmental protection. Indeed, the concept is explicit in some statements of the precautionary principle. But the idea of irreversibility remains poorly defined. Because of the flow of time, any loss is, in a sense, irreversible. On one approach, irreversibility might be understood as a reference to the value associated with taking precautionary steps that maintain flexibility for an uncertain future (‘option value’). On another approach, irreversibility might be understood to refer to the qualitatively distinctive and even unique nature of certain losses—a point that raises a claim about incommensurability. The two conceptions fit different problems. These ideas can be applied to a wide assortment of environmental and public health questions, including overuse of antibiotics, genetic modification of food, avian flu and climate change.