This chapter reviews the role of the legitimacy concept in theoretical legal studies. The author discusses how legitimacy is used as an object of inquiry to reinvent and rehabilitate international law. He then turns to the debate on legitimacy in relation to international humanitarian law and shows how technical vocabularies and doctrines are mobilized as the (de-)legitimizing tool for the behaviour of international actors. The contrast between the legitimizing of international law in the former and the legalizing of legitimacy discourses in the latter helps recall the variety of projects and agendas which inform international lawyers' engagement with international law. However, the concept of legitimacy simultaneously lays bare one common attitude among all international lawyers, namely the desire that their discourses are taken seriously and to allow international law to constitute a 'method of practical intervention in the social world'.