The Wilderness Act's National Wilderness Preservation System has grown from 9.1 million acres in 13 states in 1964, to 109,511,038 acres (758 areas) in 44 states and Puerto Rico, which represents five percent of U.S. land.

The Wilderness Act was signed into law September 3, 1964 by President Lyndon Johnson.

The Wilderness Act was primarily written by Howard Zahniser of The Wilderness Society, but its inspiration goes back to a number of influential leaders. Teddy Roosevelt created the Bureau of Forestry in 1905, and placed millions of Western forest acreage under federal protection, despite Congress limiting his efforts. As president, he created five national parks, twice the existing number, created 18 national monuments, including the Grand Canyon, and set aside 51 federal bird and four national game sanctuaries.

John Muir (1838-1914), founder and first president of the Sierra Club, championed the creation of national parks and protecting such natural wonders as the Petrified Forest and the Grand Canyon. He convinced Roosevelt to make Yosemite Valley part of a larger Yosemite National Park.

In 1924, Aldo Leopold (1887-1948), inspired by Muir, succeeded in having the USDA Forest Service recognize 574,000 acres of the Gila National Forest in New Mexico as "wilderness." Leopold and Bob Marshall (1901-1939) founded The Wilderness Society in 1935.

More than half of the protected wilderness areas are within a day's drive from such cities as Seattle, Portland, Denver, Phoenix, Los Angeles, Chicago and New York City.