Federal law generally prohibits the "use or operation of any apparatus for the transmission of energy or communications or signals by radio" without a license issued by the FCC. Anyone found operating a radio station without an FCC authorization can be subject to a variety of enforcement actions, including seizure of equipment, imposition of monetary forfeitures and criminal penalties.
There are certain limited exceptions to this rule: FCC rules authorize operation of Citizens Band (CB) radios, domestic ship and aircraft radios, radio control stations and certain other types of devices without obtaining individual station licenses. These devices must be operated in a manner consistent with the FCC's operational and technical rules. Failure to do so could be considered an unauthorized radio operation. For further information, see the FCC's consumer guide Personal Radio Services for Keeping in Touch. If you are suspicious of or are aware of unauthorized radio operations, please notify the FCC.