Updated 11/25/2020: Under the Endangered Species Act of 1973 (ESA or the Act; 16 U.S.C. §§1531-1544), the U.S. Fish and Wildlife Service (FWS) and the National Marine Fisheries Service(NMFS) (together, the Services) determine which species to “list” as “endangered species” or“threatened species,” terms defined in the Act. Species, subspecies, and distinct populationsegments (DPSs) may all be listed as “species” under the Act. Listing a species invokes certainprotections under the Act and a requirement that the Services develop a recovery plan toconserve the species. A species listed as threatened may be reclassified as endangered orvice versa. The Services may also remove a species from the list, often called delisting, if thespecies no longer meets the definition of an endangered or threatened species. The Serviceslist, reclassify, and delist species through the agency rulemaking process, guided by statutorycriteria and definitions. Persons may—and often do—challenge the legality of those final rulesthrough litigation. When such challenges succeed, the court remands the rule to the applicableService for further proceedings and may vacate the challenged rule. The case of the gray wolf (Canis lupus) exemplifies the legal issues that arise with listingand delisting species as threatened and endangered under the ESA and how FWS has addressedthem. FWS first listed the gray wolf as endangered in 1967 under the Endangered SpeciesPreservation Act (ESPA), a predecessor of the ESA. The gray wolf’s status and regulation underthe ESA and its predecessors have been the subjects of numerous FWS rules and court opinions.FWS’s gray wolf rules show how the agency’s approach to interpreting and implementing theESA has evolved and highlight hurdles that may arise with species’ status determinations. As American pioneers settled the West, hunting and other human-caused mortality, spurredby federal and state bounties, brought the gray wolf to near extinction. By the 1960s, theonly population remaining in the lower 48 states was in the northern Minnesota forests. FWSlisted the eastern timber wolf (C. lupus lycaon, a gray wolf subspecies found in Minnesota)as endangered under the ESPA. By 1976, three more gray wolf subspecies—the Mexican wolf(C. lupus baileyi), the northern Rocky Mountain wolf (C. lupus irremotus), and the Texas wolf(C. lupus monstrabilis)—were listed as endangered under the ESA. In 1978, FWS combinedall gray wolf subspecies listings. One rule listed the entire gray wolf species as endangered inthe lower 48 states except Minnesota, and a separate rule listed the gray wolf in Minnesotaas threatened. In the next few years, FWS created subspecies recovery plans that outlinedmanagement strategies and recovery criteria. In the 1990s, FWS reintroduced gray wolves tothe northern Rocky Mountains and the Southwest as experimental populations under the ESA.Protected under the ESA from human-caused mortality, which FWS identified as the greatestthreat to the species, gray wolf populations increased. In the 2000s, FWS tried on multipleoccasions to reclassify or delist gray wolf DPSs it had determined were no longer in risk ofextinction, but courts vacated many of the agency’s rules. In 2015, FWS listed the Mexican wolfseparately as endangered. In that rule, FWS adjusted the territory over which the gray wolfin the lower 48 states was endangered or threatened to exclude the Mexican wolf’s territory inArizona and New Mexico. On November 3, 2020, FWS published a final rule delisting the graywolf in the lower 48 states, with an effective date of January 4, 2021. This delisting rule doesnot affect the separate listing for the Mexican wolf as endangered.