Swaine examines the relationship between domestic constitutions and treaties. He notes that states frequently seek to interpose reservations, understanding, declarations or other “treaty conditions” that seek to change the international legal effect of a treaty for the state. The chapter considers three ways in which conditions based on constitutional claims can describe the relationship between the domestic and international domains: by establishing walls, windows or doors, paying particular attention to the “reservations dialogue” described by the International Law Commission in its Seventeenth Report on Reservation to Treaties. That dialogue reveals different dispositions toward such conditions, including as between the United States, which has employed them heavily, and European states, which have been the most innovative in assessing them. While that dialogue continues to evidence the walls, or barriers, that persist between constitutions and treaties, it remains possible to renovate.