In this paper, we use institutionalisation theory to analyse the legitimacy of schools identifying themselves or being identified by others as 'international'; identify aspects of institutionalisation theory that could be developed; and argue that an institutionalisation perspective should be more central in educational organisation theory. International schools are an appropriate form of educational organisation to analyse from an institutional perspective. The rapid growth of different types of International Schools with new rationales raises concerns about their legitimacy relative to traditional norms. In applying institutionalisation theory to International Schools, a number of relevant issues arise: the significance of the institution's primary task and institutional work on it; the centrality of affect in schools as institutions; and the very different ontological bases for the three pillars of institutionalisation; the significance of the 'student dimension' in institutionalisation, the nature of the members of the institution, their dominant mode of interpretation, their motivations and their personality; and the role of teachers' professionalism in legitimising schools. We argue that those responsible for establishing the legitimacy of their schools, will be least challenged by ensuring compliance with the regulative pillar, and most challenged by ensuring conformance to the cultural-cognitive pillar.