It takes an act of Congress to designate an area as Wilderness. Federal land management agencies are required to recommend to Congress lands they believe qualify for wilderness designation.  The Wilderness Act of 1964 directed the Forest Service, National Park Service, and Fish and Wildlife Service to survey their roadless lands for possible Wilderness designation on a state-by-state basis.  The Federal Land Policy and Management Act of 1976 directed the Bureau of Land Management to do the same. 

Congress then decides which areas to designate as Wilderness through the legislative process, and then the bill must also be signed by the president.  The original Wilderness Act designated 9.1 million acres of Wilderness across the country, primarily in the West.  Several subsequent large Wilderness bills also designated lands across the country.  Since the late 1980's, however, wilderness bills have tended to be state-specific or area-specific, such as designation of Oregon's Opal Creek Wilderness in 1998 and Steens Mountain Wilderness in 2000.

In order to be designated as Wilderness an area generally has to meet the following thresholds:

• The area must be 5,000 acres in size or larger.
• The area must generally appear to be natural and human presence is relatively unnoticeable.
• The area should offer the opportunity for primitive and unconfined recreational activities like camping, hiking, and cross country skiing.
• The area should provide opportunities for solitude.
• The area may also contain features of  ecological, geological, scientific, educational, scenic, or historical significance.

Some wildlands that do not meet the criteria for suitability as Wilderness can be protected as national parks, national forests, national monuments, national seashores, national recreation areas, national conservation areas, national wildlife refuges, wild and scenic rivers, and national scenic and historic trails.