A practical test for factual causation: would the damage have occurred but for the defendant’s act or omission? If not, the conduct is a factual cause; if it would have occurred anyway, causation fails. The but-for test is a case law formulation (not generally defined in legislation) used across negligence/delict, professional negligence, nuisance, misrepresentation, breach of contract (damages), and criminal law. The claimant/prosecutor must prove it on the balance of probabilities.Key features and practice points:- Identifies a necessary condition for loss, but does not by itself establish legal causation; scope of duty, remoteness/foreseeability, novus actus interveniens and apportionment may still limit liability.- Works straightforwardly with single causes; complexity arises with concurrent or overdetermined causes (e.g. multiple sufficient causes or evidential uncertainty).- Courts have recognised limited departures where strict but-for proof is impossible or inappropriate, such as material contribution to harm and (in some industrial disease cases) material increase in risk; specific contours depend on jurisdiction and context.Usage is broadly consistent across England & Wales, Scotland, Northern Ireland and Ireland. In the UK, certain asbestos/mesothelioma rules have statutory and common law modifications; in Ireland, analogous principles are developed in case law. The test is central to pleadings, expert evidence, and dispositive applications on causation.