The incorporation doctrine is a constitutional doctrine that makes selective provisions from the first 10 amendments to the U.S. Constitution, also known as the Bill of Rights, binding for all states.
Prior to the early 20th century, the Bill of Rights had only applied to the federal government. However, in 1925, the Supreme Court began incorporating the Bill of Rights through the due process clause of the Fourteenth Amendment, making it apply to the states as well.
The incorporation doctrine ensures state governments are held to the same standard as the federal government when it comes to protecting a citizen's constitutional rights.
Thanks to the incorporation doctrine, state governments must respect a citizen's freedom of speech, religion and assembly, as well as the separation of church and state.
The incorporation doctrine has only allowed for selective incorporation and not full incorporation, meaning not all aspects of the Bill of Rights have been applied to the states.
The Second (freedom to bear arms), Third (freedom to quarter soldiers) and Seventh (right to jury trial in civil case) Amendments have not been incorporated, nor has the portion of the Fifth Amendment that calls for a grand jury indictment.