The Library of Congress Subject Headings (LCSH) are used by most libraries in the United States to help users find resources in their collections. For the last century, librarians have worked to make LCSH more inclusive and accurate by replacing offensive or racist terms with language that more fully reflects the identities and experiences of diverse populations. This article explains the purpose and function of subject headings and discusses the process by which they are continually updated. The main emphasis of the article is the history of debates and advocacy associated with that process, with a particular focus on the long campaign to change the former subject heading "Illegal aliens."