Are you attached to the principles of the U.S. Constitution? How do you prove it—do you feel it, or just know it? What role does it play in your daily life as a citizen? Ever since one of the first acts of the U.S. Congress, the Naturalization Act of 1795, applicants for citizenship have been required to demonstrate that they are “attached to the principles of the [C]onstitution of the United States.” This requirement has been at the forefront of fierce debates in U.S. constitutional history and, although it has had limited usage after WWII, it has recently been brought up again. In 2021, President Biden announced a new bill (Citizenship Act 2021) which if passed by Congress would facilitate the pathway to citizenship for 11 million undocumented migrants that would need to show attachment as part of their naturalization requirements. Attachment requirements have also mushroomed in other liberal democracies, which have had the U.S. model in mind when designing their naturalization procedures. This Article is the first to present a systematic updated legal analysis of the attachment requirement in U.S. constitutional law and citizenship policy from a comparative perspective. The Article tracks the historical roots of the attachment requirement since the American colonies to date, demonstrates the controversies and disputes over its essence, and assesses its underlying theory, purpose, content, and methods. Overall, the Article provides normative insights, comparative lessons, and historical contexts to one of the most fundamental questions of the political community—who belongs, under what conditions, and why?