The process of nullification may refer to:

• Declaring a law to be unconstitutional and have the chance to be nullified or invalidated
• Declaring a law to be null or void in a jurisdiction, or refusing to enforce a law.
• The legal theory that a U.S. State has the right to nullify, or invalidate, any federal law which that state has deemed unconstitutional.
• Jury nullification, a legal term that refers to a jury's right to deliver a verdict in contradiction to written law.
• Body nullification, the practice of removing body parts.
• Physical nullification, a hypothesized process which results when ordinary mass/energy encounters "negative" mass/energy. Not to be confused with annihilation which involves anti-matter. For more information on negative matter, see the exotic matter article.

Nullification and attempts at nullification in the US

• The Kentucky and Virginia Resolutions nullified the Alien and Sedition Acts in Kentucky and Virginia.
• The Hartford Convention, in which New England Federalists nullified Thomas Jefferson's embargo and even considered secession from the United States of America.
• The Nullification Crisis in the United States of America, in which South Carolina passed legislation legalizing its invalidation of objectionable federal laws.
• Georgia's nullification of the Treaty of Hopewell, a treaty made by the US government with the Cherokee Nation. Georgia confiscated Cherokee land and improvements, violating the treaty.
• Refusal by Arkansas to enforce the US Supreme Court's Brown v. Board of Education 1954 school desegregation ruling.
• Perhaps the most notable current example is the nullification of US drug prohibition law by over a dozen states which have legalized medical marijuana.
• Arcata, California was the first city to pass an ordinance that bars city employees (including police and librarians) from assisting or cooperating with any federal investigations under the PATRIOT Act that would violate civil liberties. Other cities and localities have acted similarly.
• In February, 2004, the city and county of San Francisco, California, deemed the California statutory ban on same-sex marriage to be in violation of the equal protection clause of the California Constitution. Mayor Gavin Newsom directed the city-county clerk to issue marriage licenses to same-sex couples.