Negative actions, like intentional omissions or refrainments, seem to be genuine actions. The standard metaphysical theories of action are event-based: they treat actions as events of a special kind. However, it seems that many (and perhaps all) negative actions are not events, but absences thereof. This is the first book-length treatment of the problem of negative action. It surveys the recent literature, and shows how the problem is rooted in interconnected issues in metaphysics, the philosophy of action, and the philosophy of language. In particular, it connects competing views of the ontology of negative actions to competing views of the semantics of 'negative action sentences', and develops unique ontological and semantic theories to solve the problem. It provides a comprehensive picture of the nature of negative actions, our thought and talk about them, and their place in a theory of action.