Abstract: We say that a person's act caused harm if the harm would not have occurred had the person not committed the act. This meaning of causation (sometimes called causation in fact or "but for" causation) along with notions of "proximate causation" are considered in this encyclopedia entry. It is a basic characteristic of tort law that liability is not imposed unless the defendant caused harm in a relevant sense. The chief question addressed here is how this feature of tort law advances the social, instrumental ends of the legal system.