Most areas of the Appalachian Trail do not require fees or permits, but a few do. Know before you go.
Entrance or parking fees are collected at some national and state parks that the Trail passes through. Permits are required of overnight site users in the Great Smoky Mountains National Park (with a small fee) and Shenandoah National Park (no fee). A free permit is required in Baxter State Park for all A.T. long-distance hikers. Some overnight sites, especially in New England, require fees. (See below for more details.)
Regulations are set by the local land-managing agency.
Listings of these agencies can be found on ATC's site here and on the National Park Service site here.
Several hundred miles of the A.T. in all states from Virginia through Maine are located on what are known as National Park Service Appalachian Trail corridor lands. These are generally all A.T. lands outside of national, state, and local parks and forests. Comprehensive regulations for these lands can be found here.
Camping and fire regulations along the A.T. can be found here.
Certain activities, including large-group use, events, and commercial filming,
(See "special uses" below.)
All overnight hikers, including thru-hikers, are encouraged to participate in a voluntary registration system, ATcamp.
Registering will help you avoid crowded areas and will help preserve the Appalachian Trail.