This article aims to reconsider the controversial issue of the lawfulness of unauthorised humanitarian intervention. After providing a definition of humanitarian intervention and outlining its legality under contemporary international law, it examines the most common arguments raised in the literature for the purpose of justifying unilateral humanitarian intervention. The analysis covers such topics as the powers of the UN General Assembly to pass resolutions on the use of force, the theories on implicit or ex post facto authorisations by the Security Council, the text of the UN Charter, customary international law, as well as an alleged conflict of peremptory norms of international law.