Doll Modification is any sort of permanent change to a doll to give them different features, most often used to make custom dolls.
As American Girl only offers specific options in hair color, eye color, and skin tone, variations are the easiest way to have more unique dolls into a collection.
Official Modifications
American Girl currently only offers three modifications to dolls:
These are only available for 18" Dolls, in particular My American Girl dolls.
Unofficial Modifications
The most common types of unofficial modification are:
Unofficial modifications may result in a doll being unable to be repaired by the American Girl Hospital.