Most areas of the Appalachian Trail do not require fees or permits.
However, 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 just a few locations. Some of these, especially in New England, require fees. (See below for more details.)
A complete list of regulations for the Appalachian Trail on National Park Service 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, require permits. (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.