This article seeks to analyse the various responses given after NATO's intervention in Kosovo to the question of the lawfulness of unauthorised humanitarian interventions, i.e. armed interventions without authorisation of the UN Security Council. While some scholars maintained that such interventions are lawful, others held that they were clearly unlawful. The article includes a survey of previous incidents of armed interventions, but no clear answer to the question can be derived on the basis of these earlier cases. On this background, the article suggests that contemporary international law is unable to provide an answer to the question of whether or not unauthorised armed interventions for humanitarian purposes are lawful. Instead, attention should be drawn not only towards the formulation of certain criteria according to which the lawfulness of future interventions can be tested, but also to the identification of an institution or a procedure to handle the test.