I n 1964, the Wilderness Act established a national system of congressionally designated areas to be preserved in a wilderness condition: "where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain." The National Wilderness Preservation System (the Wilderness System) was originally created with 9 million acres of Forest Service lands. Congress has since added approximately 100 million more acres to the Wilderness System (see Table 1 ) among some 608.9 million acres of land managed by the federal land management agencies—the Forest Service in the Department of Agriculture, and the National Park Service (NPS), Fish and Wildlife Service (FWS), and Bureau of Land Management (BLM) in the Department of the Interior. Federal agencies, Members of Congress, and interest groups have recommended additional lands for inclusion in the Wilderness System. Furthermore, at the direction of Congress, agencies have studied, or are studying, the wilderness potential of their lands. This report provides a brief history of wilderness, describes what wilderness is, identifies permitted and prohibited uses in wilderness areas, and provides data on the 109.9 million acres of designated wilderness areas as of April 15, 2016. For information on current wilderness legislation, see CRS Report R41610, Wilderness: Issues and Legislation . As the United States was formed, the federal government acquired 1.8 billion acres of land through purchases, treaties, and other agreements. Initial federal policy was generally to transfer land to states and private ownership, but Congress also provided for reserving certain lands for federal purposes. Over time, Congress has reserved or withdrawn increasing acreage for national parks, national forests, wildlife refuges, etc. The general policy of land disposal was formally changed to a policy of retaining the remaining lands in the Federal Land Policy and Management Act of 1976 (FLPMA). The early national forests were managed for conservation—protecting and developing the lands for sustained use. It did not take long for some Forest Service leaders to recognize the need to preserve some areas in a natural state. Acting at its own discretion, and at the behest of an employee named Aldo Leopold, the Forest Service created the first wilderness area in the Gila National Forest in New Mexico in 1924. In the succeeding decades, the agency's system of wilderness, wild, and primitive areas grew to 14.6 million acres. However, in the 1950s, increasing timber harvests and recreational use of the national forests led to public concerns about the permanence of this purely administrative system. The Forest Service had relied on its administrative authority in making wilderness designations, but there was no law to prevent a future change to those designations. In response, Congress enacted the Wilderness Act in 1964. The act described the attributes and characteristics of wilderness, and it prohibited or restricted certain activities in wilderness areas to preserve and protect the designated areas, while permitting other activities to occur. The act reserves to Congress the authority to designate areas as part of the National Wilderness Preservation System. The Wilderness System began with the 9.1 million acres of national forest lands that had been identified administratively as wilderness areas or wild areas. The Wilderness Act directed the Secretary of Agriculture to review the agency's 5.5 million acres of primitive areas, and the Secretary of the Interior to evaluate the wilderness potential of National Park System and National Wildlife Refuge System lands. The Secretaries were to report their recommendations to the President and to Congress within 10 years (i.e., by 1974). Separate recommendations were made for each area, and many areas recommended for wilderness were later designated, although some of the recommendations are still pending. In 1976, FLPMA directed the Secretary of the Interior to conduct a similar review of the public lands administered by BLM within 15 years (i.e., by 1991). BLM submitted its recommendations to the President, and presidential recommendations were submitted to Congress (see " BLM Wilderness Review and Wilderness Study Areas "). The 90 th Congress began expanding the Wilderness System in 1968, as shown in Table 1 . Five laws were enacted, creating five new wilderness areas with 792,750 acres in four states. Wilderness designations generally increased in each succeeding Congress, rising to a peak of 60.8 million acres designated during the 96 th Congress (1979-1980), the largest amount designated by any Congress. This figure included the largest single designation of 56.4 million acres of wilderness through the Alaska National Interest Lands Conservation Act. The 98 th Congress enacted more wilderness laws (21) and designated more acres (8.5 million acres in 21 states) outside of Alaska than any Congress since the Wilderness System was created. The 112 th Congress was the first Congress to designate no additional wilderness acres. Including the Wilderness Act, Congress has enacted more than 100 laws designating new wilderness areas or adding to existing ones, as shown in Table 1 . To date, the 114 th Congress has designated three new wilderness areas. The Wilderness System now contains 765 wilderness areas, with approximately 109.9 million acres in 44 states and Puerto Rico, managed by the four federal land-management agencies, as shown in Table 2 . The agencies have recommended that additional lands be added to the Wilderness System; these lands are generally managed to protect their wilderness character while Congress considers adding them to the Wilderness System (see " Wilderness Review, Study, and Release "). The agencies are studying additional lands to determine if these lands should be added to the Wilderness System. However, comprehensive data on the lands recommended and being reviewed for wilderness potential are not available. The Wilderness Act described wilderness as an area of generally undisturbed federal land. Specifically, Section 2(c) defined wilderness as A wilderness, in contrast with those areas where man and his works dominate the landscape, is hereby recognized as an area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain. An area of wilderness is further defined to mean ... an area of undeveloped Federal land retaining its primeval character and influence, without permanent improvements or human habitation, which is protected and managed so as to preserve its natural conditions and which (1) generally appears to have been affected primarily by the forces of nature, with the imprint of man's work substantially unnoticeable; (2) has outstanding opportunities for solitude or a primitive and unconfined type of recreation; (3) has at least five thousand acres of land or is of sufficient size as to make practicable its preservation and use in an unimpaired condition; and (4) may also contain ecological, geological, or other features of scientific, educational, scenic, or historical value. This definition provides some general guidelines for determining which areas should, or should not, be designated wilderness, but the law contains no specific criteria. The phrases "untrammeled by man," "retaining its primeval character," and "man's work substantially unnoticeable" are far from precise. Even the numerical standard (5,000 acres) is not absolute; smaller areas can be designated if they can be protected, and the smallest wilderness area—Wisconsin Islands Wilderness in the Green Bay National Wildlife Refuge—is only 2 acres. These imprecise criteria stem in part from differing perceptions of what constitutes wilderness. To some, wilderness is an area where there is absolutely no sign of human presence: no traffic can be heard (including aircraft); no roads, structures, or litter can be seen. To others, sleeping in a camper in a 400-site campground in Yellowstone National Park is a wilderness experience. Complicating these differing perceptions is the wide range of ability to "get away from it all" in various settings. In a densely wooded area, isolation might be measured in yards; in mountainous or desert terrain, human developments can sometimes be seen for miles. Section 2 of the Wilderness Act also identified the purposes of wilderness. Specifically, Section 2(a) stated that the act's purpose was to create wilderness lands "administered for the use and enjoyment of the American people in such manner as will leave them unimpaired for future use and enjoyment as wilderness, and so as to provide for the protection of these areas, the preservation of their wilderness character, and for the gathering and dissemination of information regarding their use and enjoyment as wilderness." These criteria also contain degrees of imprecision, sometimes leading to different perceptions about how to manage wilderness. Thus, there are contrasting views about what constitutes wilderness and how wilderness should be managed. In an attempt to accommodate contrasting views of wilderness, the Wilderness Act provided certain exemptions and delayed implementation of restrictions for wilderness areas, as will be discussed below. At times, Congress has also responded to the conflicting demands of various interest groups by allowing additional exemptions for certain uses (especially for existing activities) in particular wilderness designations. The subsequent wilderness statutes have not designated wilderness areas by amending the Wilderness Act. Instead, they are independent statutes. Although nearly all of these statutes direct management in accordance with the Wilderness Act, many also provide unique management guidance for their designated areas. Ultimately, wilderness areas are whatever Congress designates as wilderness, regardless of developments or activities that some might argue conflict with the act's definition of wilderness. The Wilderness Act and subsequent wilderness laws contain several provisions addressing management of wilderness areas. These laws designate wilderness areas as part of and within existing units of federal land, and the management provisions applicable to those units of federal land, particularly those governing management direction and restricting activities, also apply. For example, hunting is prohibited in most NPS units but not on most Forest Service or BLM lands. Thus, hunting would be prohibited for the wilderness areas in those NPS units but would be permitted in Forest Service or BLM wilderness areas, absent specific statutory language.