Public necessity describes situations where the state, public authorities or, occasionally, private individuals interfere with property or other rights to avert a grave and imminent danger to the community, such as fire, flood, epidemic or serious disorder. It commonly arises as a defence or justification in tort, delict, criminal law and emergency powers. In England and Wales, Scotland and Northern Ireland, the concept is largely developed in case law rather than by a single statutory definition, and overlaps with powers under civil contingencies, public health, policing and planning legislation. Examples include destruction of property to stop the spread of fire or disease, or emergency entry onto land in the public interest. Similar principles are recognised in Ireland, informed by case law and constitutional considerations of the common good and proportionality. Key legal features include: (i) a genuine public, not merely private, danger; (ii) urgency; and (iii) a proportionate response reasonably believed necessary to address the threat. Public necessity may limit or exclude liability, affect compensation, and inform judicial review of emergency decision-making across the UK and Ireland.