In this paper I seek to explore how the idea of modus vivendi might help us to understand political legitimacy. A suitable conception of modus vivendi, I suggest, can represent a way of underpinning a viable and attractive account of political legitimacy. On my account a modus vivendi is basically a set of arrangements that are accepted as basis for conducting affairs by those who are party to them. Political legitimacy, I argue, is ultimately rooted in the judgements of those subject to it, but is mediated through a language in which claims to it are argued and assessed. The thought is that the web of operative beliefs and values in any given society, which constitute the grounds of judgements about political legitimacy, are what sustains a modus vivendi around the basic political institutions and practices. On this view, legitimate political institutions and practices incline towards a modus vivendi in that they are the outcome of an historical and ongoing conglomeration of settlements reflecting shifting and conflicting values and interests, as well relative balances of power, both currently and in the past. The marriage of modus vivendi and political legitimacy, therefore, seeks to reflect the contingent and ‘negotiated’ character of basic political institutions and practices and an understanding of political legitimacy that sees it as mediated through an ongoing and emergent discourse of argument and judgement, which remains nonetheless always vulnerable to challenge and change.