Whilst the principle of proportionality indisputably plays a crucial role in the protection of fundamental rights, it is still unclear to what extent it applies to other fields in international law. The paper therefore explores the role it plays in selected fields of public international law, beyond human rights. The examination begins in the classical domain of reprisals and in maritime boundary delimitation and continues to analyse the role played in the law of multilateral trade regulation of the World Trade Organization and in bilateral investment protection. In an attempt to explain differences in recourse to proportionality in the various fields, we develop in our conclusions a distinction between horizontal and vertical constellations of legal protection.