The doctrine of proportionality has existed as a definite strand in the fabric of international law for almost a century, even if the contours of that strand are not yet fully defined. This article summarizes the origins and history of the doctrine of proportionality generally, and then more specifically in the context of international human rights law, focusing on jurisprudence from the European Court of Human Rights. Next, we briefly consider the doctrine of proportionality in the context of the General Agreement on Tariffs and Trade, focusing on jurisprudence from the World Trade Organization. We then summarize the comparatively nascent doctrine of proportionality in international investment treaty law, focusing on jurisprudence from tribunals constituted under bilateral investment treaties, multilateral investment treaties, and free trade agreements. Finally, we conclude with potential lessons for investors, states, counsel, arbitrators, activists, and academics.