International lawyers and international relations scholars recognize that international norms change over time. Practices that were once permissible and even "normal" - like slavery, conquest, and wartime plundering - are now prohibited by international rules. Yet though we acknowledge norm change, we are just beginning to understand how and why international rules develop in the ways that they do. Wayne Sandholtz and Kendall Stiles sketch the primary theoretical perspectives on international norm change, the "legalization" and "transnational activist" approaches, and argue that both are...