There are 423 national parks in the U.S. and most of them are still free, but that doesn’t mean they won’t be overcrowded this summer. Many will require reservations.

Of the top 25 most visited national parks in the U.S., 11 of them do not charge an entry fee. And that includes the Great Smoky Mountains National Park, the Natchez Trace Parkway, the Lincoln Memorial, the Blue Ridge Parkway, the Golden Gate National Recreation Area, and George Washington Memorial Parkway.

But here’s a caution. There are still about 100 parks that have an entry fee.

If you’re planning to visit a national park this summer — entry fee or not — you must call ahead and make sure it hasn’t reached its capacity.