Over the past fifty years, proportionality balancing–an analytical procedure akin to “strict scrutiny” in the United States–has become a dominant technique of rights adjudication in the world. From German origins, proportionality analysis spread across Europe, into Commonwealth systems (Canada, New Zealand, South Africa), and Israel; it has also migrated to treaty-based regimes, including the European Union, the European Convention on Human Rights, and the World Trade Organization. Part I proposes a theory of why judges are attracted to the procedure, an account that blends strategic and normative elements. Parts III and IV provide a genealogy of proportionality, trace its global diffusion, and evaluate its impact on law and politics in a variety of settings, both national and supranational. In the conclusion, we discuss our major finding, namely, that proportionality constitutes a doctrinal underpinning for the expansion of judicial power globally. Although there is significant variation in how it is used, judges who adopt proportionality position themselves to exercise dominance over policymaking and constitutional development.