International parliamentary institutions (IPIs) are on the rise. Around the world, international organizations have increasingly established or affiliated parliamentary assemblies. At the same time, IPIs have generally remained powerless institutions with at best a consultative role in the decision-making process of international organizations. This book pursues the question why the member states of international organizations create IPIs but do not vest them with relevant institutional powers. It argues that neither the functional benefits of delegation nor the internalization of democratic norms provide convincing answers to this question. Rather, IPIs are an instrument of strategic legitimation. By establishing IPIs that mimic a highly esteemed domestic democratic institution, governments seek to ensure that audiences at home and in the wider international environment recognize their IOs as democratically legitimate. At the same time, they seek to avoid being effectively constrained by IPIs in international governance. In a statistical analysis covering the world’s most relevant international organizations and a series of case studies from diverse world regions, we find two major varieties of international parliamentarization. IOs with general purpose and high authority create and empower IPIs to legitimate their region-building projects domestically. Alternatively, IOs are induced to create parliamentary bodies by international diffusion.