Necessary or necessity may refer to:

• Need
  • An action somebody may feel they must do
  • An important task or essential thing to do at a particular time or by a particular moment
• Necessary and sufficient condition, in logic, something that is a required condition for something else to be the case
• Necessary proposition, in logic, a statement about facts that is either unassailably true (tautology) or obviously false (contradiction)
• Metaphysical necessity, in philosophy, a truth which is true in all possible worlds
• Necessity in modal logic
• Necessity good in economics

Law

• Doctrine of necessity, a concept in constitutional law
• Military necessity, a concept in international law
• Necessity (criminal law), a defence in criminal law
• Necessity (tort), a concept in the law of tort
• A necessity in contract law

Other

• Necessity., a poem by Letitia Elizabeth Landon being part of Three Extracts from the Diary of a Week, 1837.
• "Necessary" (song), by Every Little Thing, 1998
• A bathroom or toilet, in some languages (in English this is an archaic usage)
• An economic need enunciated by US President Franklin D. Roosevelt in his 1944 Second Bill of Rights
• Necessity (novel), of 2016 by Jo Walton
• Necessary Records, UK record label