Ultralight aircraft in the United States

Ultralight aircraft in the United States are not considered to be light-sport aircraft, are not regulated as aircraft, and are exempt from aircraft rules by the FAA. Instead, they are treated as powersport items and have to follow their own ruleset, FAR-103, which is the most compact aviation regulation in existence. While ultralights in other countries might have airworthiness certificates, an airworthiness certificate is not allowed for an ultralight vehicle in the USA. Any existing airworthiness certificate has to be surrendered to the issueing agency, including foreign agencies, before usage as an ultralight vehicle becomes legal.

Source: Wikipedia — Ultralight aircraft in the United States (CC BY-SA 4.0)