The phrase "separation of church and state" itself doesn't appear in the Constitution. However, the Supreme Court has interpreted several parts of the First Amendment as establishing this principle. The two key clauses are:
* The Establishment Clause: "Congress shall make no law respecting an establishment of religion..." This prevents the government from establishing a state religion or endorsing one religion over others.
* The Free Exercise Clause: "...or prohibiting the free exercise thereof..." This protects individuals' rights to practice their religion (or no religion) without government interference.
The Supreme Court has used these clauses to interpret the Constitution as requiring a separation between government and religion. The exact degree of separation is a subject of ongoing debate and legal interpretation.