The concept of nationality traditionally mediated the relationship between the individual and the state in a bygone era in which international law regarded only the latter as a genuine subject of the law; today, its international legal functions have expanded. Yet, as in the past, it remains unclear whether and how international law limits the otherwise almost plenary competence of states to confer their nationality by their internal laws in a way entitled to international recognition. After the International Court of Justice’s (“ICJ”) 1955 judgment in Nottebohm, however, lawyers began to express this limit with a kind of doctrinal mantra: a state’s national, to be a bona fide national entitled to recognition as such at the international level, must have a “genuine link” to that state. This Article critiques the genuine link theory and proposes a functional account of nationality, which, it argues, is descriptively more accurate and normatively more appealing. Nottebohm is properly read as a narrow decision in which the ICJ tacitly invoked a general principle of law, viz., abuse of rights, to prevent what it saw as a manipulative effort by the claimant to evade a critical part of the law of war. But whatever the merit of this revisionist reading of Nottebohm, the genuine link theory proves anachronistic today in view of profound changes in the manifold functions that nationality serves in contemporary international law. To illustrate, the Article suggests that the abuse-of-rights principle would also be more appropriate and effective than the genuine link theory to regulate nationality in one contemporary context that has provoked debate recently: investor-state arbitration. But the abuse-of-rights principle is no panacea. An atomized conception of nationality, which has been liberated from the genuine link theory and regulated by its functions, would best serve the policies of contemporary international law in diverse subfields.