Twenty-seven amendments have been ratified into the United States Constitution.
The first ten amendments, ratified in 1791, are known collectively as the Bill of Rights. They protect liberties including speech, religion, press, assembly, due process, and protections against unreasonable searches. Later amendments addressed major issues such as slavery, citizenship, voting rights, presidential succession, and congressional terms.
Congress has proposed 33 constitutional amendments since the Constitution became operational in 1789. Six proposed amendments were never ratified, so they are not part of the Constitution. The most recent ratified amendment is the Twenty-Seventh Amendment, which concerns congressional pay and became effective in 1992.
The amendment process is intentionally difficult. An amendment must be proposed through one of the methods in Article Five and then ratified by three-fourths of the states. This explains why the Constitution has changed far less often than ordinary federal law.