In 1952, politicians from Germany, France, Belgium, Italy, the Netherlands, and Luxembourg formed an Ad Hoc Assembly with the aim of drafting a constitution for a future European Political Community. Rediscovering this previously neglected origin of parliamentary Europe, Kari Palonen investigates the significance of the Ad Hoc Assembly for the politicization of European integration. He delves into how the debates of the assembly functioned as a project of European integration after the Second World War, interpreting it as a moment in the political theory and conceptual history of parliamentarism that opens new perspectives on the later stages of the parliamentarization of the EU.