The United States Constitution was signed at the Constitutional Convention in 1787.
Delegates signed the document in Philadelphia on September 17, 1787, after months of debate about representation, federal power, taxation, and the relationship between the national government and the states.
The Constitution did not take effect immediately upon signing. It required ratification by conventions in the states, and Article Seven specified that approval by nine states would be sufficient for the new framework to become operative.
The Constitution replaced the Articles of Confederation, which had created a weaker national government. It remains the oldest written national constitution still in use, although it has been changed through amendments and interpreted through centuries of law and practice.