Abstract: Some things, people say, are “gone forever.” But what exactly does that mean? Some losses are irreparable in the sense that nothing can be done to restore the status quo ante – or if something can be done, it is not enough (or perhaps outsiders can never know if it is). The Irreversible Harm Precautionary Principle takes the form of an insistence on paying a premium to freeze the status quo and to maintain flexibility for the future, while new information is acquired. In many settings, it makes sense to pay for an option to avoid a risk of irreversible losses. An implicit understanding of option value can be found in the emphasis on irreversibility in National Environmental Policy Act and other federal statutes, along with many international agreements. The idea of irreversibility offers a distinctive perspective on the legal concept of “irreparable harm,” a prerequisite for granting preliminary injunctions. In fact some irreparable harms seem to qualify as such precisely because they are irreversible. We can obtain new insights into the time-honored idea of irreparable harm through the lens of irreversibility, especially in environmental cases but also in many contexts, including freedom of speech, privacy, and discrimination on the basis of race and sex.