In legal research, a citator is a citation index of legal resources, one of the best-known of which in the United States is Shepard's Citations. Given a reference of a legal decision, a citator allows the researcher to find newer documents which cite the original document and thus to reconstruct the judicial history of cases and statutes. A citator can also be used to determine whether a statute or regulation has been amended, repealed, superseded, or held unconstitutional. Using a citator in this way is colloquially referred to as "Shepardizing".