The national parks of the United States have been one of the country’s most popular federal initiatives, and popular not only within the nation but across the globe. The first park was Yellowstone, established in 1872, and since then almost sixty national parks have been added, along with hundreds of monuments, protected rivers and seashores, and important historical sites as well as natural preserves. In 1916 the parks were put under the National Park Service, which has managed them primarily as scenic treasures for growing numbers of tourists. Ecologically minded scientists, however, have challenged that stewardship and called for restoration of parks to their natural conditions, defined as their ecological integrity before white Europeans intervened. The most influential voice in the history of park philosophy remains John Muir, the California naturalist and Yosemite enthusiast and himself a proto-ecologist, who saw the parks as sacred places for a modern nation, where reverence for nature and respect for science might coexist and where tourists could be educated in environmental values. As other nations have created their own park systems, similar debates have occurred. While parks may seem like a great modern idea, this idea has always been embedded in cultural and social change—and subject to struggles over what that “idea” should be.