Arbitrariness is detrimental to the legitimacy of any rule in a deep and decisive way. Yet, it is poorly understood and underexplored. This is regrettable because we need a better and more articulate understanding of it in order to use this key notion, in particular with regard to the so-called constitutional quality of legislation. In this paper, the concept of arbitrary power is analysed on the backdrop of contemporary theories of law and politics in view of establishing a conceptual framework of greater value for the scholar interested in investigating discretion and arbitrary measures in the law today than the ones currently on offer. The point is that, by viewing citizenship and migration policies from the perspective of the rule of law, new areas and dimensions of the problem of arbitrary law-making emerge. In this paper, a typology of forms of arbitrariness is sketched out and applied specifically to the analysis of citizenship policies and border control techniques.