The document that serves as the supreme law of the land in the United States is the Constitution. Ratified in 1788, the Constitution establishes the framework for the U.S. government, delineating the powers and responsibilities of each branch while protecting individual rights through the Bill of Rights. It embodies the principles of federalism and checks and balances, ensuring that no single branch becomes too powerful. The Constitution is a living document, permitting amendments to address evolving societal needs and challenges, and it remains the foundation of American law and governance.