20 photos that show the exquisite beauty of America's National Parks

"National parks are the best idea we ever had," historian Wallace Stegner wrote in 1983. "Absolutely American, absolutely democratic, they reflect us at our best rather than our worst."

This week is National Park Week, an annual tradition started in 1991 to pay homage to that "best idea" — and to call attention to the resources required to maintain it.

We've collected photos from the 20 most-visited national parks in the U.S.

Great Smoky Mountains National Park

Charles Krebs/Corbis

The National Park Service (NPS) launched a campaign earlier this month to promote the many parks, landmarks and monuments that preserve not only nature, but also American history. Read more...