This chapter lays out the theoretical framework for the study of international parliamentary institutions (IPIs). First, it argues that IPIs display few of the functional benefits that are commonly associated with the delegation of competences to international institutions. Second, it claims that the assumption of normatively committed member states of international organizations (IOs) does not explain either the weakness of IPIs, their appearance in many IOs composed of non-democratic states, or their absence from IOs with a solid democratic membership. Third, we suggest that the creation and empowerment of IPIs is better understood as a legitimation strategy that governments employ strategically in response to challenges to the legitimacy of the organization. Maintaining or improving the legitimacy of an IO is important because it enhances the stability of cooperation and prevents the disruption of its operation, and international parliamentarization is specifically useful when democratic legitimacy is the standard by which relevant audiences judge an IO. Finally, we identify six structural conditions that generate variation in the normative cost–benefit calculus of governments. These relate to institutional characteristics of the IO itself (authority, purpose, and scope), as well as its domestic and international environment (democracy, governance failure, and diffusion).