The American Conservative Union (ACU) is an American political organization that advocates for conservative policies, ranks politicians based on their level of conservatism, and organizes the Conservative Political Action Conference. Founded on December 18, 1964, it calls itself the oldest ongoing conservative lobbying organization in the U.S. The ACU is concerned with issues such as personal liberty or freedom, and traditional values, which they define as foundations of conservatism.

The ACU comprises three entities: The American Conservative Union, a 501(c)(4) organization which conducts lobbying; The American Conservative Union Foundation, a 501(c)(3) organization best known for hosting the Conservative Political Action Conference; and The American Conservative Union Political Action Committee, a PAC that formally endorses and funds conservative candidates for federal and state level offices.^{[citation needed]}

Dating back to 1971, ACU has implemented its own scoring system which annually rates politicians on their conservatism. While the scorecard was novel to conservatism, Americans for Democratic Action has utilized a liberal rubric for liberalism since 1947.

Each publication of Congressional and State Ratings contains a statement from Chairman Matt Schlapp about the philosophy guiding the ratings as one of conservatism: "We begin with our philosophy (conservatism is the political philosophy that sovereignty resides in the person) and then apply our understanding of government (its essential role is to defend life, liberty, and property)."

Unlike other congressional ratings that take positions on pending legislation, ACU Foundation rates votes already cast by lawmakers. Each rating provides a conservative interpretation of an official's view of governance. As one spokesperson for the ACU once noted, "clear-cut distinctions between liberals and conservatives [occur] if you have Crane, Ashbrook, and Kemp go a certain way and Burton goes the other".

The ACU annually rates politicians according to how they vote on key issues, providing a numerical indicator of how much the lawmakers agreed with conservative ideals. They use this rating system as a point of accountability for politicians, comparing their political rhetoric to their voting records to assess their conservativeness. Politicians are given a percentile rating, anyone with a rating of over 80% is considered to be an "ACU Conservative". These scores are often used in political science research, in news stories and in election campaigns.

ACU's most well-known event is the Conservative Political Action Conference (CPAC), an annual event organized by the ACU foundation. CPAC has an annual attendance of thousands. Speakers regularly include sitting and former presidents and other famous conservatives. CPAC 2017 featured President Donald Trump, Vice President Mike Pence, Senator Ted Cruz (R-TX), Governors Matt Bevin (R-KY), Sam Brownback (R-KS), Doug Ducey (R-AZ), and Scott Walker (R-WI) and executive branch officials (EPA Administrator Scott Pruitt and Education Secretary Betsy DeVos).

The ACU Foundation's purpose is to educate the public on conservative principles and currently has five "policy centers" which focus on different political areas. There is the Center for Arts and Culture, the Center for Human Dignity, the Center for Statesmanship & Diplomacy, the Center for 21st Century Property Rights, and the Center for Criminal Justice Reform (CCJR). These policy centers are mainly blogs which post articles regarding their topic area. The most extensive of these is the CCJR, who advocate for conservative criminal justice reform through advising governmental officials, media advocacy, and testifying as expert witnesses at governmental hearings. The CCJR focuses on two main policy areas: preventing civil asset forfeiture and increasing mental health facilities within the criminal justice system. The CCJR works with the Texas Public Policy Foundation and Prison Fellow Ministries in the Right on Crime campaign, and offers a panel at the Conservative Political Action Conference each year.