The FAA requires you to register as a “MODELER Section 336” and mark your model aircraft with your registration number in case it is lost or stolen.
The registration number must be applied to the exterior of the model.
Registration cost is $5 and is valid for 3 years. You will only have to register once for all of your RC aircraft as one registration number can be applied to all the aircraft that you have.
It is also recommended that you join the AMA (Academy of Model Aeronautics) which is an insurance policy that protects you should something occur while you are flying your RC aircraft.