The Ninth Amendment of the U.S. Constitution affirms that people have general rights that the government cannot take away; the amendment doesn't specifically name any of these rights. Some examples of modern issues that the Ninth Amendment might apply to include marriage equality and abortion.
The Ninth Amendment was introduced primarily to prevent expansion of government power by omission. It prevents Congress from automatically denying rights that are not specifically covered in the Constitution.
As such, what the Ninth Amendment actually applies to is open to interpretation. The Supreme Court originally ruled in Barron v. Baltimore that the amendment could only be applied by the federal government to federal laws, along with most of the rest of the Bill of Rights, but the Court has since cited it in striking down state and local laws prohibiting homosexual acts and the use of contraception.
Legal and academic interpretations of applications of the Ninth Amendment vary greatly. Supreme Court Justices such as John Paul Stevens have cited trends in the laws of other countries as a basis for determining what constitutes a human right subject to Constitutional protection under the amendment. An opposing view is that the amendment potentially grants too much power to the federal judiciary.