I examine the degree to which the unconstitutional constitutional amendment doctrine has been employed in the American states. One purpose is to identify the occasions and reasons why state courts have invalidated amendments and to show that rulings have focused on inaccurate ballot language, multiple subjects, procedural violations, violations of federal law, and subject-matter violations. However, and this is the paper’s second purpose, notably absent are state court cases employing the unconstitutional constitutional amendment doctrine, in the sense of invalidating amendments for violating state constitutional provisions or principles. The paper’s third purpose is to explain why American state courts have not embraced the unconstitutional constitutional amendment doctrine in a way that has been embraced by courts in other polities. A key explanation is found in the strength of the popular sovereignty doctrine and role of the public in approving constitutional changes in the U.S.