Northern Rocky Mountain wolves, a subspecies of the gray wolf (Canis lupus), were native to Yellowstone when the park was established in 1872. Predator control was practiced here in the late 1800s and early 1900s. Between 1914 and 1926, at least 136 wolves were killed in the park; by the 1940s, wolf packs were rarely reported. By the 1970s, scientists found no evidence of a wolf population in Yellowstone; wolves persisted in the lower 48 states only in northern Minnesota and on Isle Royale in Michigan. An occasional wolf likely wandered into the Yellowstone area; however, no verifiable evidence of a breeding pair of wolves existed through the mid 1990s. In the early 1980s, wolves began to reestablish themselves near Glacier National Park in northern Montana; an estimated 75 wolves inhabited Montana in 1996. At the same time, wolf reports were increasing in central and north-central Idaho, and wolves were occasionally reported in the state of Washington. The wolf is listed as "endangered" throughout its historic range in the lower 48 states except in Minnesota, where it is "threatened."

National Park Service (NPS) policy calls for restoring native species when: a) sufficient habitat exists to support a self-perpetuating population, b) management can prevent serious threats to outside interests, c) the restored subspecies most nearly resembles the extirpated subspecies, and d) extirpation resulted from human activities.

The U.S. Fish & Wildlife Service 1987 Northern Rocky Mountain Wolf Recovery Plan proposed reintroduction of an "experimental population" of wolves into Yellowstone. In a report to Congress, scientists from the University of Wyoming predicted reductions of elk (15%-25%), bison (5%-15%), moose, and mule deer could result from wolf restoration in Yellowstone. A separate panel of 15 experts predicted decreases in moose (10%-15%) and mule deer (20%-30%). Minor effects were predicted for grizzly bears and mountain lions. Coyotes probably would decline and red foxes probably would increase.

In October 1991, Congress provided funds to the U.S Fish & Wildlife Service (USFWS) to prepare, in consultation with the NPS and the U.S. Forest Service, an Environmental Impact Statement (EIS) on restoring wolves to Yellowstone and central Idaho. After several years and a near-record number of public comments, the Secretary of Interior signed the Record of Decision on the Final Environmental Impact Statement (FEIS) for reintroduction of gray wolves to both areas. Staff from Yellowstone, the USFWS, and participating states prepared to implement wolf restoration. The USFWS prepared special regulations outlining how wolves would be managed as a nonessential experimental population under section 10(j) of the Endangered Species Act. These regulations took effect in November 1994. As outlined in the Record of Decision, the states and tribes would implement and lead wolf management outside the boundaries of national parks and wildlife refuges, within federal guidelines. The states of Idaho, Wyoming, and Montana have begun preparation of wolf management plans.

Park staff assisted with planning for a soft release of wolves in Yellowstone. This technique has been used to restore red wolves in the southeastern United States and swift fox in the Great Plains and involves holding animals temporarily in areas of suitable habitat. Penning of the animals is intended to discourage immediate long-distance dispersal. In contrast, a hard release allows animals to disperse immediately wherever they choose, and has been used in Idaho where there is limited access to the central Idaho wilderness.

In the autumn of 1995 at three sites in the Lamar Valley, park staff completed site planning, and archaeological and sensitive plant surveys. Approximately 1 acre was enclosed at each site with 9-gauge chain link fence in 10' x 10' panels. These enclosures could be dismantled and reconstructed at other sites if necessary. The fences had a 2' overhang and a 4' skirt at the bottom to discourage climbing over or digging under the enclosure. Each pen had a small holding area attached, to allow a wolf to be separated from the group for medical treatment. Inside each pen were several plywood security boxes to provide shelter. For the 1996 release, one pen was relocated to Blacktail Plateau and another was constructed in the Firehole Valley in central Yellowstone. Subsequently pens have been relocated from Lamar to other areas in the park interior to facilitate releases into other geographic areas or the park or special circumstances that require the temporary penning of wolves.

2007 Data
At the end of 2007, at least 171 wolves in 11 packs and various groups occupied Yellowstone National Park. There were fewer packs than in 2006, but the remaining packs were larger and there were more non-pack wolves (e.g., loners, temporary pairs, etc.). This represents a 26% increase from 2006 and closely matches the population peak of 174 in 2003. Ten packs counted toward the breeding pair objective for the Yellowstone Recovery Area; the Hayden Valley pack lost both breeders late in the year and did not count. One new pack, the Gardner’s Hole pack, formed but did not survive until the end of 2007 (despite producing pups). This was the first year since reintroduction that no new packs formed. One radio collared wolf from Idaho dispersed into the park in late 2007.